HI Tony,
This could be a number of issues, here are a couple of things to try before spending any cash.
1: move the 641 to another alot.
2: Reseat the SCSI cables.
3: If it's got the 128MB cach upgrade module & you arent running any RAID 5 arrays try removing this module and leaving it to run off the onboard 64mb read cache.
4: Run the smart array diagnostics for your o/s and see if the board has captured any historical errors that might point to a likely culprit.
If none of these work then replacement 641/642 controllers are available on eBay these days for pennies as ar the beefier 6402's. The other less desirable option is that the nmotherboard is faulty & these still command a fair price.
Hope this helps.
Cheers
Mark