What software version are you running? the implication is you had a FR245M but replaced it with a FR245? Was it a recent purchase? Did it work OK at first and then start acting poorly (maybe went through a software update)?
Automatic updates are turned on by default, I turned mine off because the 1st watch I purchased did an update without out asking and messed up the HR sensor a few days after I bought it. I'm on 5.5 now looking though these posts to see if it's reasonable to update the software. It was 11.1 a few days back, now at 11.6 which is concerning.
I took the first watch back since it was only a few days old, got the new one, then turned of automatic updates. The first watch was version 6 software, worked fine (for my needs) version 8.x caused the HR reading to drop by 20 to 40 BPM.
