I have a Sears/Kenmore gas furnace hot water heating system thats acting erratically.

What works:

1.) The house thermostat appears to work.
2.) I hear the electronic igniter "clicking".
3.) I hear the gas supply flowing.
4.) I see and hear the gas ignite and stay lite.

What does not work:

1.) The Grundig circulator pump (which makes a humming sound when it operates) does not appear to be operating (no hum, as before).

2.) The heating system works as described above but seems to cycle on and off without water circulation - the circulator hum is no longer there. In the past, I am 90% sure the circulator pump kicked in whenever the furnace was activated by the room thermostat.

3.) MOST IMPORTANT: When the furnace is activated (by initially raising the temperature setting of the room thermostat) the igniter lites the gas and the furnace runs for a few minutes, then it shuts down. The new room thermostat setting is completely ignored and little heat reaches the room radiators. STATS: The furnace reads a temperature of about 180 degrees F with a water pressure of about 18 psi.

I suspect (but I am likely WRONG) the furnace is reaching a top temperature and then cycling off as a protective measure because the pump is not circulating the heated water to the room radiators and returning cooler water for reheating. What I cannot understand is why the pump does not operate. Also, the furnace temperature reading does not appear to be very "high" (high enough to cause a furnace shutdown) or beyond a normal Winter operating temperature.

Sorry for the long post - just trying to describe the problem.

Thanks for all your help.