This problem has started in the last few days, with a long established program. On several objects the retrieve api call fails with "there is an error in XML document"

This is happening on account, case, contact, lead, opportunity and task objects (others appear OK). Also the error sometimes "goes away", the following is a partial trace:
TaskgetRecordDetail(1): Error: There is an error in XML document (1259, 22).
TaskgetRecordDetail(2): Error: There is an error in XML document (528, 45).
TaskgetRecordDetail(3): Error: There is an error in XML document (13050, 21).
TaskgetRecordDetail(4): Error: There is an error in XML document (3048, 6).
TaskgetRecordDetail(5): Error: There is an error in XML document (11611, 6).
TaskgetRecordDetail(6): Error: There is an error in XML document (14958, 6).
TaskgetRecordDetail(1): Error: There is an error in XML document (1337, 6).
TaskgetRecordDetail(2): Error: There is an error in XML document (14857, 5).
TaskgetRecordDetail(3): Error: There is an error in XML document (9781, 6).

My routine retries up to 10 times, primarily to compensate for network errors. But here the retry gives the same error but at a different points in the xml, finally getting a sucessful response.