98 GT running cool

With the weather turning cool I noticed that my car doesn't want to get up to temp on the highway and takes a little longer to warm up. On the highway it's running around 10F low. Previous winters it always warmed up fairly quick and then held the normal temp, even on the coldest days and we're no where near that right now.

I'm thinking that the thermostat could be sticking in a close to fully open position? Reasoning for this is that at the end of the summer I noticed on the hot days that the temp would start to climb on the highway if I was running at more than 110km/hr on a 30 degree day. This is completely new as it has always been bang on. It wouldn't overheat, but would just run 10-15 degrees warm, thought at that time that maybe the rad was going bad, but it wasn't losing coolant and the weather got cooler so I figured that I'd deal with it next year, or when something happened. Seeing this now, makes me think that if the thermostat is stuck in a partial to close to fully open state then this could be the reasoning behind both of these problems.

Thoughts?

Try another thermostat (195 degree). Plus if the TS is stuck open you may be taking a hit on fuel economy. For the engine to operate efficiently the engine temperature has to get up to at least 145 degrees to switch over to closed loop mode in a reasonable period of time.

Thanks, such a cheap thing to fix, I'll be taking it to my mechanic tomorrow.