Check the vac lines and hoses going to the cruise control servo you'll see a device with two metal cables that has an electrical plug in connector that's the cruise control servo unit, also the cruise control has a more then likely black ground wire you could have a bad ground so trace it and clean he ground connection, plus the cruise control has a relay that could be acting up too.
Remember that the cruise control only works as well as the vehicle is tuned so check the air filter , perform a basic tune etc.
Hope that helps and best of luck.By the way since a comp controls the cruise control via the switches and relay etc the comp could be acting up or a switch or the wiring could have a problem and again be sure to check vac hoses, vac lines for any leaks or cracks etc too.
