Thermostat Cars temperature gage fluxuates a lot. Sometimes it hits the red and the alarm sounds but there is no smoke. Now the heater only occasionally works. Last year the garage cleaned gunk out of the radiator and it worked for many months.

Possible defective temperature sender, sensor. Thermostat could be stuck closed. Extreme case headgasket blown.

Sounds like they put the wrong temperature of thermostat in you car. If they put in a thermostat that had a lower temperature range than the original. This causes it to open to early and circulate cold water which would give the symptoms you are talking about.

PUT A GOOD THERMOSTAT. AND GASKET. AND REST

sounds like a restricted radiator.. the thermostat has 2 x jobs to maintain the temperature and to restrict the flow-so by removing the thermostat you actually went backwards- meaning the coolant will flow to fast thru the radiator and will not have time to be cooled down prior to returning to the engine block. since your heater helps to cool it down my guess(educated) would be a restricted radiator..

low coolant sensor is probably just dirty. cold air could also indicate n air bubble in the line. could also be thermostat. try that first.

The water pump impeller is plastic, and is known to crack. I would replace the water pump.

You probably need to bleed the cooling system properly.

Open the cooling system bleeder screws.

Slowly fill the cooling system with a 50/50 coolant mixture until coolant runs freely out of bleeder screws.
Close the cooling system bleeder screws
Install the coolant pressure cap
Start the engine.
Run the engine at 2,000 - 2,500 RPM until the engine reaches normal operating temperature. (midway on gauge)
Allow the engine to idle for 3 minutes.
Shut the engine off.
Allow the engine to completely cool.
Top off the coolant as necessary.
Run engine again at 2000 - 2500 RPM until the engine reaches normal operating temperature.
Allow engine to idle until cooling fans come on. It should come on with the gauge about 3/4 of the way to red. - if it gets to where is touches the red, shut down immediately.
When the fan comes on, put your hand behind the fan and observe the temperature of the air coming from the fan. It should feel hot. If it is blowing cold air, the coolant is not circulating through the radiator like it should. This could be caused by lots of things including an improper/defective thermostat, corroded water pump impeller, or a blown head gasket.

A thermostat stuck in the open position can cause both of these problems

Is it just the gauge or is the engine coolant temperature varying? Does the heater seem to vary in temperature when it is on.
It could be a couple of different things.
One could be a malfunctioning temp sending unit, it has lost its calibration.
Two, your thermostat is cycling open and closed from a weak return spring, worn out.
Make sure coolant level is correct and the mixture is 50/50.
Let me know.
Regards,

Hello!

Sounds strange. If the water pump was really replaced, then they should have replaced the thermostat (which often causes such problems), also only correct thermostat must be used. (best bought from the dealer)
The only other thing I can suspect is the head gasket damage, but I would double check the thermostats first.

Hope this helps!