I’ve had this exact same problem for the last week with an old Wahoo Tickr. Works fine for about 45 minutes, then freezes. I’ve tried replacing the strap and battery, neither of which resolved the issue.
Strangely enough, it worked prefectly on a 2 hour outdoor ride this morning, so I’m reluctant to pull the trigger on a new purchase if it’s not needed.
I was wondering if there is a bluetooth issue with my phone, but I’ve had the same problem using an iPad as well, which suggests the fault it with the HRM (or the TR app).