Wrist heart rate sensor inaccurate until I restart the watch

I have been wearing my FR45 while lifting weights, not to track the activity, but merely to time my rest periods and to keep an eye on my heart rate. To check my heart rate after a set, I hit the "down" button to display my current heart rate reading.

Recently I've noticed that it seems really inaccurate. For example, it will show: 62, 64, 63, 61, 64, etc. immediately after a set. I know that my HR is much higher than that just by feel, and holding my hand on my chest I can feel my heat beating much more rapidly. So I will power the watch down and back up, hit the down button again, wait a few seconds for it to get a reading, and it then shows a reading 30 to 40 beats higher, e.g.: 102, 104, 101, 99, 100, etc., which seems much more accurate!  This second reading is coming 30 seconds or so after I have completed my set, so the fact of the matter is that when I took the first reading, it was actually probably 15 or 20 beats higher, probably in the 120-130 range.

I know that accurate wrist HR is dependent on having the watch positioned snugly and correctly on my wrist. But in this case, that is not the issue here, since I'm doing nothing other than cycling the power--not moving the watch on my wrist in any way--and suddenly the HR reading is accurate. This would suggest that it is not a sensor issue, but instead is some kind of software issue.

I also know that wrist HR is generally unreliable when exercising--when I run, I always use a chest strap--but when I'm not actively moving (e.g. when resting after a lifting set), it should be accurate then.

Has anyone else run into this issue? Any suggestions on how to fix? It's somewhat frustrating.