which T-stat did you buy? FWIW, the first time I went to order a stat from the local dealer they asked me "which one do you want? 180* or 195*?" I kind of didn't know, so I did my own search on the GM parts sites & found that it looked like, to me anyway, GM offers two. Looking thru all our stuff over at the netreg site, it seemed the car came with the 180* as OE.
Also, in the distant past Stant had a miss-box happen and people got a ford stat(?) in the LT5's stat box. The results were not very satisfying at all.
When you say "driving around" do you mean highway or secondary streets? I ask because the observed behavior of the gauge seems just like my gauge behaves if I'm doing the around town, 40 mph or less thing...no B to B traffic, but no above 50 stuff. If you are describing 50mph or better as in highway use, then yea something is wrong for sure as the gauge should be reading about 1/2, ya know around either side, but not toward the 3/4 mark. Only other reason I can think of is that the gauge's sensor picked that moment to go south on you, or it's air bound.
Tom