Cooling issue, but this time it's to cold...

Are you sure you have enough water in the system. If your water is low, and yes it can be low and still show in the rad, you will get exactly the problem you are seeing. The car will warm up and you will get heat until the stat opens and your temps go down. I f you are low on water the temp will read low even though the engine is very hot and the heater will not work.

Jack