I am using Data Loader to import records from on old SF system to a new one. I am familiar with the nature of the Invalid Cross Reference id error and the reasons for it however I don't understand why I am getting this error with the upload of data into the opportunity object/table. 1/2 of the records upload successfully but 1/2 dont.

I have checked the ID fields that I use in the data upload (ie RecordTypeID, PriceBookID, AccountID, etc) and I cannot see an obvious trend with the rejected records compared to the successful ones. The AccountIDs are new and valid and as for the other ID fields, I can see examples of them in the 'success' records as well as the error records, so the ID values cant be invalid if they worked for numerous other records. The only ID field that can be considered unique is the AccountID which I am associating these payment (opportunity) records with but they are all AccountIDs from the new SF system which I have linked to using vlookup in excel. If they were invalid then all the upload records should be invalid not just half of them.

Does anyone have any idea why this error is occurring on half my opportunity upload records?
any advice is much appreciated.