H2 Tranny Slip ???

[solarflare]

Not sure if it is actually slipping, but I have noticed at around 55mph, and going up a gradual incline, instead of shifting it seemed to slip. I have noticed this a couple of times, in the past week.
Today I also noticed that my tranny temperature seemed a little warm. When it's around 40 degrees outside, the tranny never gets above 130 degrees.....even when it's 90 degrees out it never gets above 160 degrees.
But today I noticed that it hit 172. I was only idling, and usually it will cool down when idling....checked the level, and it was perfect.
not really sure what is happening, and was wondering if anyone has experienced this? 2008 H2 with 6.2V8 and 6 speed tranny