I dunno what sensor you use, but that may be the cause of your troubles.
In O2 (99% pure, and dry), your cell mV should be
0.99/.209 x AirMv.
A few mV above this ideal value is good. A few mV below is not good.
Hope this helps.
I have an unreliable VR3. It seems to reset itself every time I have a deco obligation, or when inside a cave

.