Inaccurate HR

Happened twice when doing Z2 runs on the treadmill.

The first time, my HR kept going up and up and reached 170 bps within a few mins when I know 100% it was 140 at most (I do the same run all the time). It was off by +30.

The previous time it dropped from 140 to 110 within a few mins of my run and stayed at 105-115 throughout the entire run. It was off by -30.

I've only had my 955 solar for 2-3 weeks and already thinking of ditching it. Thoughts?