Where are you taking your temps, and what are you reading them with?
I am using an IR hand gauge. I have (had) 2 of them (different models), and I would some times get a 10 F difference between them, making me also question my test equipment. I think I trust the IR more than a thermocouple probe, as the fans blow too much air across the test surfaces I am checking and cool the probe too much while I am trying to get a reading. I trust bulk part radiation (IR) a little more (at least currently). My dash gauge says I am 190-200 F (rear head sensor) when my IR says the T-stat housing is at 225 to 235 F. I am sure I have no air trapped, so one of them is lying, or there is a huge differential between the coolant temps in the two locations on a Renix, and or one of the two temperature measurement devices are out of wack, or both.
My 87 and my 89, use to run cooler, 10 F cooler with the AC running (in say 85-90 F ambient, R-12 setup ), but the AC also turned on the electric fan. That was running with 160 and 180 T-stats.
This year with the R-134a upgrade (or downgrade), and the 100 F heat wave, it was, has been getting 20 F hotter when I turn on the AC, but I hot wired the fan a few weeks ago, so the comparison is a little bit different.
I suspect something in my valve train was out of wack (sticking lifter noise) for a long, long time, that made mine run cooler the last 4 years, as my daughters 89, with the new head, runs better, more power, but also about 20 F hotter than mine use to run. Now all of a sudden, my MPGs went up, and it started running hotter even with out the AC on. Now it runs a little hotter than my daughters, and hers started running cooler after her new head broke in. So many variables, makes it hard to isolate cause and effects. Also hard to figure out what really is normal with these beasts, and I have 4 of them!