Today, I burned the IC803 Eprom with the patch v1.1 of the ko4bb download. Alas, the problem persists.
But here is an interesting observation: I checked the data files I had collected over time and indeed, the jump occurs always in the order of 15min! See the attached plot of today, with the patch applied. Horizontal unit is 10s per bin, makes about 800s, gives 13min...and .there was a bit time before the data were taken. So indeed it looks as if it is somehow related to the AZ.
Edit: of course it may be that it just coincidentally, a certain temperature is always reached after ~15min given constant ambient temp. I will run the whole test overnight at low temp to distentangle time vs temperature, once for all.
Edit2:
I added a plot where I re-initialized a few times the 7081 a few moments after a drop. As one can see, the value before the drop is resumed, until the next drop happens. The drops appear typically around 15min after but not precisely, there are variations. I can hear some relays click after 15min, and assume that this is due to AZ. However, I didn't observe any coinicident jump. Moreover, as you see there was a drop to zero which was remedied by itself, and in the end the whole thing crashed and got stuck at output zero.
So there seems to be a circumstantial evidence that AZ is involved but the timing is not 100% accurate.
I feel about giving up -- if the single jump after ~15min is 100% reproducible, as it looks, it can be taken into account with the calibration (which is messed up anyway). From the various thermal defects I encountered so far the 7081 must have has a pretty hot pre-history..
Thanks for all!