Leaky overheating hot & cold heat on my Pontiac ! Help

You have a leak in the system somewhere. Probably just a hose. Take a close look around where the steam is coming from and you will likely spot it. When you find the source, change the hose and any others that appear old or do not feel flexible at the same time.

I thought that too!! But unfortunately it's not steaming anymore, but the temperature gauge is still going up and down and blowing cold air out of the heater on and off, the leak is very strange, when the car is off, coolant disappears, but when I start it up the coolant doesn't really leak bad. I don't think it's the heater core bc the inside of the car is not leaking. I tried to find the leak, but it doesn't seem like any hoses are the source.

The coolant is supposed to retract into the radiator from the overflow reservoir when the car cools but the reservoir is never supposed to get empty. There are 2 lines on it - cold fill and hot fill. Depending on the engine temp, make sure it is full to the proper line. If it is not, your coolant is low and you should add more up to the proper line. If you do that, your "leak" may re-appear (or disappear if it was really low and the steam was causing the cap to exceed its rating instead of coolant exiting to the reservoir).