I've got a trigger on a custom object that is not working.

Here's how it should work:

A user goes into a standard sales-record type Account record.  User creates from that account a record on a custom object called Customer Usage.  Once that record is created the cu record is associated to the account with a simple lookup field to the account record to which it belongs.  What's important to know here is that the account object has a lookup field called Referring_Partner__c.  This lookup field is a lookup to the account object itself, and is used to hold the value of a partner-record type account name.

What I want my trigger to do is, when a customer usage record is created or updated with a value in its First Positive Invoice Date field (it's a datetime field), I want that datetime to populate in field on the related account record Partner First Gross Add field. 

The problem is that I want the trigger to update the Partner First Gross Add field on the account that is related to the account from which the customer usage record was created, in other words, the trigger should update the partner account record that is related to the sales account from which the cu record was created.

Example:

Account X = Partner Account record

Account Y = Sales Account record

Customer Usage Z = Customer Usage record

User creates a customer usage record called Z with Account__c field value of Y and First Positive Invoice Date of 6/2/2011.

Upon insert of Z, record X should be updated with a value of 6/2/2011 in the Partner First Gross Add field.

The trigger below does the above, but, what it does is update record Y with 6/2/2011 in the Partner First Gross Add field.  What I want the trigger to do is update record X with 6/2/2011 in the Partner First Gross Add field.

How do I do this?  Please help.  I've spent probably 15 hours on this and can't figure it out.

trigger CustomerUsage on Customer_Usage__c (before insert, before update, after insert, after update, after delete) {

//-----
// Account Updates Processing
//------

if ((Trigger.isInsert || Trigger.isUpdate || Trigger.isDelete) && Trigger.isAfter) {    
     
set<Id> AccountIds = new set<Id>();
 
  if(trigger.isInsert || trigger.isUpdate){
    for(Customer_Usage__c p : trigger.new){
      AccountIds.add(p.Account__c);
    }
  }
 
  if(trigger.isDelete){
    for(Customer_Usage__c p : trigger.old){
      AccountIds.add(p.Account__c);
    }
  }
 
  map<Id,DateTime> AccountMap = new map <Id,Datetime>();
 
  for(Customer_Usage__c q : [select Id,First_Positive_Invoice_Date__c
    from Customer_Usage__c where Account__r.Referring_Partner__c IN :AccountIds  order by First_Positive_Invoice_Date__c asc limit 1]){
      AccountMap.put(
      
      (ID)q.get('Account__c'),
      (Datetime)q.get('First_Positive_Invoice_Date__c')
      
      
      );
  }
 
  List <Account> AccountsToUpdate = new List <Account>();
 
  for(Account a : [Select Id,Referring_Partner__c,Partner_First_Gross_Add__c from Account where Id IN :AccountIds]){
    Datetime FPID = AccountMap.get(a.Referring_Partner__r.Id);
    a.Partner_First_Gross_Add__c = FPID ;
    AccountsToUpdate.add(a);
    

  }
 
  update AccountsToUpdate;



}