I have a 1995 pontiac grand prix. The temperature gauge goes up during idle when i stop at stop signs or traffic lights, but then goes back down when i start driving. What can be the problem?

Check your cooling fan operation. When you're moving, you have more air flowing across radiator, helping it cool. Your engine is also revving higher, so the water pump can move more coolant through engine. If it's getting hotter sitting still, then the cooling fan may not be working, or the water pump may be worn. It's also possible you have a partially blocked radiator, they do gunk up on the inside leaving too few water passages to do the job. So look at those 3 things: cooling fan(s), water pump, and radiator. Wouldn't hurt to go through entire cooling system and make sure it is up to snuff, given the vehicles age.

It could be a bad fan relay. If the relay goes bad then the fan may not operate when you don't have the A/C on. Try running the car at idle in park. Turn on the A?C and see if the fan is running. If it is then the fan motor is ok. Then warm the car up and see if the fan kicks in when the temperature gets to the second line on the temperature gauge below the red line. If the fan doesn't come on it;s probably the fan relay. It's in the fuse box under the hood on the passenger side. I believe the relays are Fan 1 and Fan 3. Here is the diagram: https://www.2carpros.com/questions/2006-pontiac-grand-prix-relay-location