03 Monte Carlo LS heating problems

I am having problems with no heat coming out of the vents even if the car sits for 20 minutes to warm up. But when i am on the highway it blows out some warm air then when I come to a stop it blows cold air again. It is about 10 degrees out and i really need heat. Also I noticed the temperature guage fluctuautes. When the engine is warmed up it will usually sit at about 1/4 the way up. Then after a while it creeps up just above 1/2 and then creeps back down to 1/4. This shouldn't be the thermostat because i just had it changed about a month ago along with cooling system flush. Please help nobody will reply in Monte forum. Thanks.

okay my mother stopped at mechanic after work and he had to put 1 whole gallon of coolant in. so now i know theres a leak somewhere. i looked near thermostat area and it looks "wet" so i'm assuming that when he replaced the thermostat it wasnt tight enough or gasket is not sealing. i also felt upper rad. hose and it was pretty warm. any ideas on places to look for leaks. engine is 3.4 liter and it has about 124000 miles on it. thanks.

well i found the leak and it's under the throttle body where the thermostat is sitting. i am going to take it back to mechanic and tell him to take it out and reassemble it and make sure the gasket is on right and whatnot. i would think people would've read this and replied with suggestions but oh well. dont need anymore help. thanks anyway.