have an '89 that did this exact same thing -push in clutch RPM's go up to 2k or so, makes for hard mountain driving as was my case. I'll tell you now that it took me almost 2 years of it coming snd going with everybody suggesting all kinds of high dollar parts replacement and tricky vacuum/ electrical type measuring ohm and volt values while engaging or disengaging of every little part 1 could imagine, wrong! It's simply a matter or very closely examining all of the vacuum lines that become very brittle after a few hundred thousand miles my case 275k for now till I rebuild the tranny. When I say closely I mean it ! mine was a tiny hairline crack running along the length of a tube all of 1/4" in length, the reason it came and went was due to the shacking around of the car and motor leaving the crack opened up more or not at all. I did replace a few of the lines with simply putting windshield washer hose pieces over the areas I thought the cracks were. It seemed like it did it but nope just as un predictable as it was it would return over and over until I ripped out the clump of 4 or so plastic lines that all joined together at a square 5 or so to 1 type connection 1 had a cap on the unused nipple. this is how I saw it because now I could see it from every angle under a lighted magnifying glass and done fixed! 2 weeks later my transmission bites it and that was 3 years ago I still say I'm going to fix the tranny but in the mean time have a 2002 GT to drive until I'm in the mood to start that job. I'm sure that I'll get to it before I need to replace the 2002 with a newer model but only time will tell.
