Outlook Rules Keep Stopping

I have some rules running Server side on Outlook 2002 & Exchange 2000, on 1 client they just stop processing for no reason this has happened 4 times now. I have checked all event logs and re-created the mapi profile & rules from new and still 1 or 2 clients just stop, all of the other clients are working fine, same rules, XP Build, Worksation. Does anyone know how to troubleshoot this further.

Thanks.