I have the same problem with my 88. It sat in a field for bout 5 years before i bought. And now its my everyday driver and soon to be restoration project. But I have the same problem and cant seem to figure it out. I've replaced 2 thermostats and they were 192s and the car heats up a little while idling but when i drive it goes back down to bout 120. Need some help before it gets too cold! Lol