I wasn't sure under which section to post this because I am not entirely sure what is causing it, but here goes.

I just bought a 92 Plymouth Acclaim and it seems to be perfect: it shifts cleanly, starts right up, little body rot/rust. The problem is when I take it up past 55mph the speedometer starts hopping up and down wildly. It is difficult to get an accurate measure of speed at this point, and also when the speedometer reaches 120 (the vehicle limit) its kicks violently. Could anyone tell me what the problem might be here, how I could fix it, or how much it would cost to be fixed roughly? Thank you very much!

I would investigate the speed sensor....they aren't that expensive and you could install yourself

The rev limiter may be activated at 120mph. What this does is shuts cylinders down so that the car cannot be revved and higher to protect itself from burning out. Check your RPM when you are at 120mph. Or it could be speed activated. depends.

I concur, you speed sensor may be jacked up. Easy to replace.
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