91 GMC Jimmy 4dr 2wd 4.3L engine 700r4 tranny, 3.08 gears, . 160000 miles. Runs drive great, except for one problem. Full throttle it shifts great 1-2, and another good shift 2-3(at about 60mph). Then at 75mph, it kicks back down into 2nd gear. If I let off to about 2/3 throttle it shifts back up into 3rd and it's fine past that in third(if I leave it at about 2/3 throttle). What is going on? It's done this for over a year now and never gotten worse or better. It does it about 75 percent of the time--once it a while it stays in 3rd at full throttle. Shift quaility is excellent on all shifts(1-2 will bark the tires sometimes). I'm not real concerned about fixing it right away, since it hasn't gotten worse and I drive on 60mph roads most of the time. Just curious what could be the problem. Around a year ago I did a fluid change and installed a cooler--same problem and same excellent shift quality before and after. Fluid was clean before and after the fluid change and is still clean. Sorry for the long post, trying to be specific.