Hello! Did you ever find the source of the leak that you had in summer? Now would be a good time to open the system up, do a complete flush. I have a suspicion that the stop leak has found its way into the heater core. You say fan is on almost right away? Are you running in defrost? If car is equipped with a/c, compressor will run, and fan may come on? even with heat called for.
1. Try to find leak, so it can be repaired properly.
2. Drain and flush everything thouroghly.
3. I would replace thermostat since system is open.
Good luck! This should give you a good starting point.