2003 Other Toyota Models Cruise Control Malfunction

I have a 2003 Matrix. When I first engage the cruise control, it will hold speed, and the accellerate/coast functions work okay. But it will shut off after about five minutes. Upon subsequent attempts, the cruise stays on for shorter and shorter durations. A Toyota mechanic made some adjustment to the brake pedal (tightened it, I believe), but the condition still persists. I understand there are many areas of a cruise control system to be diagnosed, but are there one or two areas that are most LIKELY the problem - so I can indicate same to the Toyota tech guy when I take it in again, and hopefully shorten the diagnostic time for what could be an expensive repair? Thank you.