One of the least expensive checks is to go through all the exterior light bulbs and the 3rd brake light bulb and make sure they are all the same brand and wattage, preferably German bulbs. The speed control switches off when your brakes are applied, and if there is evev a tiny 'strange' ohmage on either brake circuit (each circuit shares a brake bulb with other bulbs - look at the info inside the lid of the fuse box)it can affect cruise control operation. This may not be your problem, but is an easy and dirt cheap possible fix.
Also, there was a recall on throttle linkage bushings. If yours wasn't done that could make your linkage difficult for the unit to control, if I recall correctly.
The system has a controller and an amplifier, and I don't recall the order to check them in or replace them if they have a problem. Check the search feature - this has been discussed a lot over the last couple of years. I am sure some of the techs will answer with even better ideas about what is going on. Good luck, and let us know what you find.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E