I've had some older cars that when the speedo bounced at speeds, it was from dry cable inside the housing, but if you have an electronic speedometer, it would be from a speed sensor that may be going out or the grounds/connector pins needs cleaned due to bad contacts.
I would try some "Electronic Spray Cleaner" and disconnect all the connectors first and spray to clean off any electronic corrosion that builds up on electronic pins. Then I would smear some "Dielectric Grease" on the pin connections to help keep out moisture/corrosion.
If you can get to any grounding cables/wires I would take apart and wire brush/sandpaper away any rust to get a good clean surface contact. Just another something, to eliminate a possibility.
As to the cruise control, it can be broken wires inside the steering wheel column, a wire(s) that had become disconnected at the cruise control unit itself, blown fuse, vacuum diaphragm rubber cracked and won't hold vacuum, vacuum hose going to cruise control unit from engine disconnected or cracked and need replaced, Pull cable from cruise unit to carb/throttle arm popped off. Many things to look at, depending on the type of cruise unit model.
Sounds like something easy as the cruise worked recently, but going through the eliminate this/that does take time. Many times the hardest part is just finding where the factory put the part, to fix.
Del & Jerre w/Isis our English Bull Terrier
The Cougar's Den
1984 Fleetwood Prowler "E"