Heating problem

I recently began with less and less heat output from my 95 Lumina 3.1 car. First garage told me the head gasket was shot and put in a container of quick plug in the antifreeze but I left looking for a second opinion. Second place I took it to replaced the thermostat and the heater core. Same problem came back after the heat did work for a day or so. Third garage replaced the thermostat, readjusted the heater core and flushed the system. They have now done this twice. Same problem running around in about 0 degrees F there is moderate heat but not the way it was a month ago and certainly not the heat output from all the previous years I've owned it.
Is there anyone that could tell me what else to look for? Hoses are hot but I noticed that on the dash gauge that there are fairly large swings in temperatures from being at a stop to starting to drive. I suspect that a circulation probem but the rad shop says they tested pressure and its okay and the system was bled so I don't know what else to do.
thanks

Ive had the same problem recently.

nothing done on the car yet . any suggestion where to start?

glycol full

new pump 3 months ago ( very noisy)

I recently began with less and less heat output from my 95 Lumina 3.1 car. First garage told me the head gasket was shot and put in a container of quick plug in the antifreeze but I left looking for a second opinion. Second place I took it to replaced the thermostat and the heater core. Same problem came back after the heat did work for a day or so. Third garage replaced the thermostat, readjusted the heater core and flushed the system. They have now done this twice. Same problem running around in about 0 degrees F there is moderate heat but not the way it was a month ago and certainly not the heat output from all the previous years I've owned it.
Is there anyone that could tell me what else to look for? Hoses are hot but I noticed that on the dash gauge that there are fairly large swings in temperatures from being at a stop to starting to drive. I suspect that a circulation probem but the rad shop says they tested pressure and its okay and the system was bled so I don't know what else to do.
thanks

I started having the same problem with my heater not working. and my coolant not staying in the overflow tank. i had it checked also because of fear of a leak. it was a leak and it was in the intake gasket manifold. which luminas and 3.1 l engines are known for from gm. i had stop leak put in and the problem hasnt occured since. however if it was your gasket head blown, im pretty sure your car wouldnt even run.

You might have a weak water pump. Get the car in operating temperature and make sure the top hose is hot and firm. If the hose is "mushy" then the pump is weak. Also, i've seen the same problem by simply a bad radiator cap. Radiator caps should be replaced every couple years period, otherwise it can cause heating/cooling issues.