OS/400 V4R3M0 SQL0501 Error

We have a probem with SQL’s on our AS/400 (V4R3M0). When running some programs using SQL we get an SQL0901 & SQL0501 Error.
This problem only happens around 1 in 5 times but never the less is a big problem.
This has only started happening in the last few days and as far as i know no changes have been made to the system (except a disk upgrade).
We have contacted IBM and installed the latest PTFs and this made no difference. Even IBM seem to be at a loss.

Please email: simon.bowyer@s3t.co.uk if you have ans ideas. Thank you.