Just something to look into... Perhaps others have also had this problem.

I have been having similar type problems in an app that has been running for years - tables not opened in work areas, and variables not found. I found that they installed a VOIP telephone system just before the problems started. Reboot all the phones, and the problem disappears for around 3 to 4 days, and then it creeps in again.. And strangely, it seems to be the same tables and variables each time. Reboot the phones, and all is fine again...