98 GT overheating?

I have a 98 GP GT with 80 K on it, that the temp guage "normally" is at 185 during driving conditions. Recently it has started to go up to 210 when at idle and returns to 185 on the highway. I have noticed this now that the weather has warmed here. ( Wisconsin ). I have not noticed this happening before in the 2 years i've owned it. I was reading in a repair manual that if your temp gauge rises and feel the TOP radiator hose is not hot then it is a bad thermostat. I felt my top hose and it was hot. Did I read wrong or could this be another problem? Also I have read that if replacing the T-stat you have to replace the gasket + O ring as well with a 98. When changing out the T-stat would it be advisable to do a flush of the system and replace the coolant that is in it? I have looked at the coolant and it is red or pink, NOT green or blue as i've seen before. Someone told me that it is Toyota coolant. Thanks for any and all input.

The coolant in your car is GM Dex-cool, and GM has used it since 95'ish. I would change the t-stat since it's cheap and easy to do, and you might as well drain as much of the old coolant as you can, and top it back off with new. Dex-cool is claimed to last 5 years/150,000 miles, but it will start to sludge when exposed to air as it ages and gets dirty. If your coolant level is low, look around the base of the throttle body for signs of cooant leakage, as there are known issues with the upper intake plenum. The EGR stove pipe was poorly designed, and will melt and deform the plenum causing a coolant leak. If you have this issue, and let it go too long, it can leak so much coolant into the engine it could hydrolock, so check it out.

yeah that happend to me and had to change my plenum and plugs and it was fun fun.