02 coolant problems

Hi i have a 02 impala and i have had the intake gasket replaced.The temp hand act erratic.it goes all they way to the hot then i have to cut the car off for 2min then it got back to normal.I about to replace the thermostat;the water pump has been replaced.The low coolant light stay on even if the radiator and the overflow is full.What could be the problem! With the low coolant light and the erratic temp hand?And the heat goes from hot to cold when it is about to over heat.

3.4L or 3.8L engine?

Air in the system. Bleed it out, you'll be fine.

thanks and it was a 3.4 and can i bleed the system out myself if so how!

There's a small brass hex head bolt near the thermostat that you can open while the engine is running to let the air out. When the bubbling and spitting stops and fluid starts draining out consistently, close the bolt. Be careful to note how much you turned the bolt head to open it and turn it back by about the same amount to close it. The goal is to not overtighten the bolt - which is very possible with brass if you're not being careful...

I need to have the car running correct.

Yes, the car must be running. Start the bleeding process when the car is cold. I think the valve is 7mm, but I'm not sure.
When the coolant starts to warm up, it will bubble out the brass bleeder valve. The valve is the highest point in the coolant system, so the air will eventually be pushed out, along with some coolant. It won't lose more than a quart or so, in my experience, but it depends on how much air you have in there. When you have a steady stream of coolant, close the valve. Be careful, as the coolant and valve will be hot by that time.

Thanks for all your help guys.It seem to be doing a lot better now I drove the car around to block a few times.I will know when i go to work in the morning.And i also notice sluge gunk on the radiator cap.Should i get it flushed out.

Thanks for all your help guys.It seem to be doing a lot better now I drove the car around to block a few times.I will know when i go to work in the morning.And i also notice sluge gunk on the radiator cap.Should i get it flushed out.
if the coolant has sludge and if radiator cooling tubes have sludge you will have to flush system or risk overheat engine damage. if dexcool use distilled water . whenever dexcool is mixed with non dex cool coolant sludge will form. usually starts with heater not working then engine temp too high. sludge goes to heater core first.if you have good heat now you are probably ok............