There are two systems in this cruise control. The first is the electronics, which is pretty dependable. But, the electronics control vacuum to the servo, and the vacuum is what actually pulls the throttle open. There is a weakness in the vacuum part of it.
There is a hose from the vacuum diaphragm that goes to the brake pedal bracket, and then a valve of sorts on the bracket itself. The idea is that pressing the brake pedal release the vacuum and stops the cruise control action. The valve at the brake pedal is an odd design, it basically has the end of the hose and then a stopper on the bracket. The distance between the two is a friction adjustment. So, over time, the hose, which adjusts, can back away and cause intermittent and then complete failure of the cruise control. The way to fix it is to get up under the dash at the brake pedal, find that hose and adjust it down so it makes better contact with the stopper. This is hard to describe in words, but once you see it, you will get the idea.
I have had to do this 3-4 times in my 10 years of ownership. If it's the problem, you just need a flashlight to see it and use your hands to adjust it. That would be my best first guess.
