(a) Cruise control. Unfortunately a hotbed of problems.
Known problems :
(1) Cruise control amplifier located under kick panel under steering column. Can develop solder breaks on circuit board. Solution is to clear off the lacquer coating from the circuit board with acetone (or finger nail cleaner) and resolder any suspicious looking joints.
(2) Hall effect sender on the back of the speedo.
(3) "Bouncy" speedo suspect speedo cable either kinked or not fed stright into instrument.
(4) Cruise control actuator - under the AFM in the engine bay. internal electrical problems.
(5) There is a small grommet on the accelerator controls that can go missing. The cruise actuator will not connect to the accelerator linkages smoothly.

New stereo. Do you have a fuse in the electrical live input? - maybe not connecting properly. Faulty earthing. If you install a modern head unit then you should run a new power supply.

Windows. Most likely one of the window switches in the console is giving a short circuit.