QLOCKSIN Error when processing large numbers of messages

FIrstly I am a bit of a newbie at this.

We need to make payments using an interface to our bank. Currently paying vendors is working OK as there are a maximum of a few thousand payments in each payment batch. However when we test paying 10,000 employees the interface is failing.

We are using the RFFOEDI1 transaction to generate an idoc for each payment and a summary idoc for the batch. There are 7 batches that are generated with 7,500 payments in the largest batch.

We initally had a problem with the maximum number of nodes in the SWPA transaction. This has been increased from 50,000 to 100,000 and we don't have this problem now.

However we are getting a problem where the queue is stopped with the following entry in the Inbound Queue: Function Module: STOP, TDI: QLOCKSIN1111111111111111, and Program: RSWWDHEX_INTERNAL.

Restarting the queue seems to get the messages processing again, but how do I find out what is causing this "error" and the queue to be stopped.

Thanks

Ian