Sporadic Baseboard Heating Issue:(

[Russell Kahn]

have a multifamily property with a single boiler hot water baseboard heat. Each apartment has a pop top valve with a thermostat. In one studio apartment the heat sporadically stops working. Once 2 years ago the pipe burst. The water will stop flowing sometimes, usually on cold days here in Wisconsin like 5 degrees or below. At the boiler each apartment has its own piping, it's not a single loop system. I've changed the valve and bled it many times. Yesterday it stopped when the apartment was still 60 degrees. There was some cold air coming through the wall by the inlet and outlet (where the pipe goes into the floor) so I used foam to seal those cold areas. In areas, the pipes go through the room inside the concrete flooring. If the boiler is working and the building and other apartments are warm why would only one apartment have this issue? Is it the cold draft hitting the pipes? Any ideas why 0-3 times a winter it stops working?