I replaced a two-wire battery powered tstat with an Hvac powered one (without battery back up). So the 'Common' line was unused until now.

After about 10 minutes of calling for heat, the tstat loses power and turns off and comes back on a minute or so later. It doesn't seem to be a tstat issue. last time it went dead, I pulled it off and checked the wires with a meter, it read ~3V or less for a while and came back up to normal.

This only happens during heat-call and not other times. What's going on here? Is there something that kicks in after a few minutes and chokes the tstat?

Here is how the lines are connected from the boiler to the tstat:

TR -> Common
TH-TR -> Return
TH-> W (heat call)