Vacuum Time Error

We are having repeated Vacuum Time Errors with our Tuttnauer Class B, Prevacuum Steam Sterilizer, Model #: 3870 HSG WS.  We've verified the Water Tanks are properly filled, the Drain line appears to be not clogged, and the exhaust appears to be clear.  We are seeing this at random times, and now with random cycles, i.e. Bowie Dick Test, Vacuum Test, Wrap 121..  I can attach Cycle Printouts here for evaluation.  Thanks for your help/advice.