[Fat Cat]

Ok, for the last several weeks I've noticed that the heat blows significantly cooler everytime I come to a stop and the car sits at idle. Soon as I pick up speed, it instantly blows hot again. Today I had an interesting situation to go along with this. I left work and stopped for gas. Car was fully warmed up. I go to leave and it's blowing ice cold air, no heat. I get on the highway and the temp is rising, so I go to pull over and the temp drops to normal and I continue on my way. Stop one more time at Home Depot. Soon as I slow down for the stop lights and driving into the mall, starts blowing ice cold again and temp goes way up.

Checked the coolant level and it is full, however I noticed some drips on the reservoir coming from under the cap. First I was thinking thermostat, but didn't make sense since this happens so instantly. Since the air coming out is ice cold, I'm thinking there can't be hot coolant going through the heater core..... This could mean system is plugged up somewhere.... Possibly the heater core itself or the radiator even? Would anyone agree that I'm on the right track with this?

[salem1912]

Fat Cat was the rad full or the reservoir? sometimes a weak cap can push coolant out of a hot radiator to the reservoir but not allow it to draw back in to the radiator when it cools.

[Lightfoot]

Yes, it does sound to me like there is not enough coolant in the block. Sending unit for the gauge is in an air pocket most of the time, and can't give a real reading. Then when the small amount of coolant boils up and hits it, it is way hot.
Probably the only reason you have been getting by without seriously overheating the engine is because the winter cold.

If you can reach them, try gripping the heater lines next time the heater is blowing ice cold. If they are hot, then you could also have a plugged or restricted heater core.