orp meter acting funny

[wasp9166]

i have a pinpoint orp meter, it started acting funny lately , after maybe 2 years.....................i replaced the probe but have same problem, i calibrated it with 400mv solution, and this is what it does..............when i turn off power to my tank the orp reading jumps up about 20mv, then slowly falls back down to what it was reading originally when power is reapplied......................if i turn power on and off, repeatedly, it will continue to climb by 20 mv or so with every power shutoff................have you ever heard of anything like this? i thought maybe its stray voltage , i have a grounding probe in sump , and i stuck both probes of a volt meter in water and get no reading.............????