Coolant temperature

When our 1998 van is at idle, the temperature climbs to the end of the white range at the gage (hooked up my scan tool and got a reading of 239°F) but never goes in the red, both fans are running - at 95°F outside it took about 10 minutes for the cold engine to reach that temperature. When diving, the temperature hovers around 204°F. As soon as I slow down, the temperature climbs again.
Question: Could it be a problem with the T-stat or would it be the water pump?
The T-stat was replaced during the IMF job about 3-1/2 years ago and has a total of about 170,000 miles on it. Water pump is more than likely the original (purchased the van used in 2005).

I would start by cleaning the exterior of the radiator. At the shop I use compressed air and blow from the engine side of the radiator out to the bumper. You would be surprised at how much debris can get stuck to the front and in between the fins of a radiator. Sometimes you can find a spot where you can see in between the condensor and the radiator and see the debris built up on the the front of the radiator

Update - took off the radiator cap today and found the radiator low on coolant (actually pretty low). Put in some coolant and saw a light spray of coolant being propelled up by the belt. It seems to be coming from the water pump which I will replace in the next few days.

Sorry, I assumed you had checked the coolant level!?!?!?

Sorry, I assumed you had checked the coolant level!?!?!?
Only checked the level in the overflow tank - which was OK and didn't think about it much more at first, but had a nagging feeling and removed the cap to find the radiator being low. I always thought that if the fluid gets low that the overflow tank will take care of that - guess I was wrong. At least that job doesn't look too bad and the pump is pretty inexpensive.