I am getting a unique SOAP response from a SOAP query request I am doing from Salesforce to an external system (AS400). 

This response is a product of a query I send to the external system asking what has changed and to provide certain fields that have changed. If you notice the first response is a map of the fields that will be in the response. "<a1>POLICY.NUMBER</a1> <a2>CLIENT.CODE</a2>"..ect..

I have tried to use XMLStreamReader and the DOM class, but I'm having trouble actually getting to the Item and corresponding A1,A2,A3 elements. Any ideas on how best to handle this response? 

Also, there will be cases where there is more than one record that is returned so I will have to find every "File" that is returned (other than the index File) and make sure to assign all the values associated with that given File. Then continue looping through onto the next File and assign the other fields / values to that record. 

Example of XML Response: 

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <soap:Body>
        <PassThroughReqResponse xmlns="http://amsservices.com/">
            <PassThroughReqResult><?xml version="1.0"?>
<PickResponse>
  <Files>
    <File sagfile="WORK.7864470">
      <Item sagitem="WEBSERVICE.TAGS*WORK.7864470">
        <a1>POLICY.NUMBER</a1>
        <a2>CLIENT.CODE</a2>
        <a3>EFF.DATE</a3>
        <a4>EXP.DATE</a4>
        <a5>COV</a5>
        <a6>INS</a6>
        <a7>TERM</a7>
        <a8>POLICY.STATUS</a8>
        <a9>BILLING.METHOD</a9>
        <a10>31</a10>
        <a11>32</a11>
        <a12>33</a12>
        <a13>POLICY.STATUS</a13>
        <a14>NEW.REN</a14>
        <a15>CHG.TIME</a15>
      </Item>
    </File>
    <File sagfile="WORK.7864470">
      <Item sagitem="579615">
        <a1>TESTINGNEW</a1>
        <a2>ZIMJO1</a2>
        <a3>08/08/16</a3>
        <a4>08/08/17</a4>
        <a5>ARA</a5>
        <a6>CER</a6>
        <a7>A</a7>
        <a9>Agency Bill</a9>
        <a14>NEW</a14>
        <a15>12:06:46</a15>
      </Item>
    </File>
  </Files>
</PickResponse></PassThroughReqResult>
        </PassThroughReqResponse>
    </soap:Body>
</soap:Envelope>

Here is a code snippet that I have hacked to work, but it is not efficient and often times hangs due to CPU usage. This continues on for each A# tag. 
 

//Capture SagittaID
        List<String> sagittaID = new List<String>();

        //List values to be returned 
        List<String> sagittaResponseTag = new List<String>(); 
        sagittaResponseTag.add('Item');
        sagittaResponseTag.add('a1'); //Client Code
        sagittaResponseTag.add('a2'); //Policy Number
        sagittaResponseTag.add('a3'); //Effective Date
        sagittaResponseTag.add('a4'); //Expiration Date
        sagittaResponseTag.add('a5'); //Cov
        sagittaResponseTag.add('a6'); //Insuror
        sagittaResponseTag.add('a7'); //Term
        sagittaResponseTag.add('a8'); //Policy Status
        sagittaResponseTag.add('a9'); //Billing Method
        sagittaResponseTag.add('a10'); //Written Prem
        sagittaResponseTag.add('a11'); //Written Agency Prem
        sagittaResponseTag.add('a12'); //Writer Producer Prem
        sagittaResponseTag.add('a13'); // Policy Status
        sagittaResponseTag.add('a14'); // New or Renewal


        for (Integer i = 0; i < sagittaResponseTag.size(); i ++) {  

             
            while (reader.hasNext()) { //while there are more XML events
                if (reader.getEventType() == XmlTag.START_ELEMENT) { //if this is the opening tag
                    String localName = reader.getlocalName();
                    String listTag   = string.valueOf(sagittaResponseTag.get(i));
                    System.debug('Inside Loop  :' + i + 'Local Tag Name  :' + localName + 'Does this equal :' + listTag);
            
                    if (sagittaResponseTag.get(i) == reader.getlocalName()) {
                        if (sagittaResponseTag.get(i) == 'Item' && reader.getLocalName() == 'Item') {
                            for (Integer a = 0; a < reader.getAttributeCount(); a ++) { 
                                if (reader.getAttributeLocalName(a).equals('sagitem')) { 
                                    theSagittaIdValue = reader.getAttributeValueAt(a); 
                                    sagittaID.add(theSagittaIdValue);
                                    System.debug('Found SagittaID: ' + i + '  Value :'+ theSagittaIdValue);
                                     
                                }  
                            } 
                        }  
                    } 
                } reader.next();  //advance to the next XML event

                if ('a1' == reader.getlocalName()) {
                    System.debug('Found A1');
                    while(reader.hasNext()) { //while there are more XML events
                        if (reader.getEventType() == XmlTag.END_ELEMENT) { //if this is the closing tag
                            break; //exist the loop
                        } else if (reader.getEventType() == XmlTag.CHARACTERS) { //if this is the content between the tags
                            theSagittaCodeValue = reader.getText();
                           // grab the content
                            if(theSagittaCodeValue != null) {
                                sagittaClientCode.add(theSagittaCodeValue);
                                System.debug('Sagitta Client Code: Insert' + ':' + theSagittaCodeValue );
                            }
                        }
                        
                        
                    }

                } reader.next(); //advance to the next XML event