just bought my 93 sc300 with 200,000 miles on it. was running normal when i first got it but after sitting a few months some issues started to arrise. When I shift into Drive the cars idle will bob from 600rpm to almost dying then bounce back up. If i shift into park the idle will go back to normal. Also when I get on the freeway the car will accelerate fine til I hit 60ish. THe revs will continue to go up but the car wont accelerate. Feels like a slipping clutch.

Is my tranny ****ting out? Im planning to go to 5 speed anyways but Im wondering if anyone has any insight on my issue. thanks