FWIW. My 2004 T1N has always been OK for heat. The other day I watched things a bit closer to get some timing. About 30 F ambient. A 1/2 mile back street drive to the expressway entrance ramp. About 2 miles to the toll barrier 55 mph limit. 65 mph limit from there.
Started van @ approx. 7:42 a.m.
Temp hit 145 F about 7:48 a.m.
Temp hit a couple ticks below 180 F about 7:56 a.m.
About 8:07 a.m. still just under 180 F. I reduced the cabin blower fan speed and adjusted the vents because it was starting to get a bit hot.
Not very scientific, but gives an idea of how mine works. Same basic results in colder weather. It just takes a little longer to hit the temperature benchmarks.
I think Gears hit on a good thing to check with the jiggle valve. Anything which allows too much coolant to by-pass the stat will keep operating temperatures down. Any by-pass leakage will not affect summer driving temperatures so much.
I also think the 12 minute change out time and $75.00 parts described by Mr. Schmancy is better time spent and cost effective than a visit to the shop for evaluation. I'd try that first.
Hope this does some good. AP/vic