V6? V8? Which engine?
It's a K34 RPO car. They're electronic "transducers" of sorts, not the old style speedometer driven transducers. A lot of the time you can point to 2 major causes....the cruise wand or the servo unit itself. It may be a vacuum leak. Seriously doubt it's the speed sensor (little green AC thingie that sits in the dash near the speedo). They hardly ever go bad.
But....there's also a brake lamp switch with cruise. Again- fat chance, but can't ever rule it out. If it's sticking you could be getting a "disengage" condition when you don't want it to. GM p/n 25524845.
Do you have delay wipers? If so, you would need a delay cruise turn signal lever, GM p/n 25031456 or equivalent. If not, you get one without the delay, GM p/n 25031457. Just a note- They're the exact same switch/lever functionally, the delay or non-delay lettering imprint on the barrel is the only thing different. So if you have a good used one laying around, it doesn't matter about the imprint as the delay or non-delay switch inside the column is what does all that.
The servo you need gots a chain on it I believe. For a Chebby V8, it's one of these numbers- 25031884, 25074084, 25074085, 25074632, 25074649
If you have the 231 V6, the p/n for the servo is GM p/n 25074630.
