My 1998 Pontiac Grand Prix is overheating, please help

Check to make sure the cooling fans work, there's a cooling temp sensor and a relay that turns them on

You just described the symptoms of a early head gasket failure. Some on here will or like to say no do not tell him that it is probably this or that but this is in fact the exact symptoms I had for months before it finally started to blow white smoke/steam out of the tail pipe and started t miss a little then ran hot more often and later got harder to crank.

For your fan isn't running it is #getting hotwhen it stopped because no air is passing over the motor it is not overheating on the highway because you were going 60 miles an hour and air is flowing over the motor check to see if your fan is kicking on

It was 2 years ago but the other answers are simply idiotic. If fans aren't working right and causing overheating, overheating will happen regardless of speed, outside temp or so on. I've never heard of early head gasket failure! One way head gasket failure happens is when antifreeze enters a cylinder then it will burn off in the form of exhaust smoke which lowers your coolant level therefore car runs hotter because some of your coolant burnt out through the cars cylinder or got trapped in there thus not allowing motor to turn because liquid doesn't easily flow by your valves as air does and if you actually continue driving, foot on floor and can't build up speed I'm sure, think about oil and water, they don't mix! Antifreeze will push past your rings, and push out oil and leave cylinder without lubrication therefore metal to metal locking the motor.... OMG... If idiotic is not clarified to you yet, sell, trade, get rid of your vehicle and start collecting bus schedules!! If that point was clear, an open or removed thermostat will create steady flow of coolant instead of keeping temp of af in block regulated. It will also not a allow pressure to properly build. Lower rpm means slower flow giving af time to get warmer. Or car could need a good flush (not drain) and refill to remove any thing blocking the coolant flow. AF hindrances are likely more noticeable at idle with more coolant moving at higher speeds. Off chance but possible faulty sensor. But my most likely reason (and this has also happened to me) Water pump! It's a good idea to change thermostat (+/-$10 and easy) radiator cap (+/-$13 removed during water pump change, replace with new) and use fresh and compatible antifreeze, and consider bleeding air out as part of process that might need repeated for a day or two. Whew, that was fun but now I'm..(get antifreeze with denatonium benzoate a bittering agent so becoming poisoned by consumption is less likely).. done. Thank you