pH/DO Module PCB Assy Boards?

Hi - I have a few Bioflo 115s that all have the same issue, pH drift and stability is all over the place. If I put the pH probes in pH 7 cal buffer and wait for the raw value to stabilize, then I enter the 7.0 and then click the Set Zero button, the value goes to 7.0 for a bit of time then the raw value starts running once direction or the other, until the value stabilizes again. Then I rinse and repeat... enter 7.0, click Set Zero, 7.0 appears, then drift. Changing the probe or cable doesn't help. I've been trying to troubleshoot with no support from Eppendorf. I've installed new and/or slightly used pH probes, changed the pH cable with known working cables, and used a multimeter to test the volts coming in and going out of the power supply unit/EMF power line filter. These things all test AOK. So, all fingers are pointing to the pH/DO module PCBs being faulty. I removed one board and visually inspected under a magnifying light. I didn't see any popped chips or black or burnt marks. Visually AOK. My question: What else can I test to further prove that the pH/DO Module is faulty? I can order these boards new, but they are quite expensive so my supervisor wants me to prove to him that they are the cause. Any help or thoughts would be welcome. Thanks.

Equipment: New Brunswick Scientific - BioFlo/CelliGen 115