1998 grand prix over heating

assuming that you are not getting heat from the vent, my guess is that something is blocking coolant flow to or from the heater core (supplies heat for interior comfort)

when you replaced the thermostat did you burp the system?

Check your heater core and the water pump. Also, it is possible that there is an air bubble in the system. Pressurize it to clear the air and improve water flow.

how do you get all the air out or bulp it like you say

I have the same exact problem. but I replaced the therostat. and the temp still goes up when the car idles. then it goes down when I take off..