I have no idea, but I just set two mooshimeters to log my bedroom radiator input/output temperatures with ntc-sensors (salvaged from heated floor thermostats, both did show about 40 Kohm at room temperature, so probably 33Kohm@25degC). First one at 10 s interval and other at 1 minute. 125Hz/64smpl (auto) sampling rate.

How long you actually let it sit before checking it out? Log for a week and screwed at ~24h or log for a day and things went crashing way before that? Did you connect to your mooshimeter while your experiment was going on? I just like to know more, so i could reproduce your setup.. My meters showed 100 and 86 percent for batteries, did you have full batteries to start with?