The only things you didn't mention is the throttle cable connected to the vacuum unit and the vacuum hoses.
There is an adjustment proceedure that "must" be followed to make sure both the hand throttle cables and the cruise throttle cables work together.
If the cruise cables are to loose, for example, the cruise will not stay on....cause the speed will drop to far before it can engage. Mine drops about 5mph before the vacuum unit catches up. The system is a bit slow.....but there are a couple of simple proceedures... I just hold throttle until the cruise catches up...about 4 seconds. I need to adjust my cables...just haven't got there yet!
Check the vacuum hose between the pump and actuator. Check for loose connections at same area. Losing vacuum will make the cruise drop off and not hold speed.
Last, you may need up taking the computer out and check for bad solder joints...usually where the connector comes into the computer. If the connectors are corroded, or the circuit boards have a bad solder joint...the unit will be intermittant or worse...
Suggest you start in one area and go back over each piece...and then each system..(electrical, vaccum, cables).
Please report back what you are finding....lots of help here...lots of experience!
Good luck,
david