The dealer can record the electronics while driving it as I used to do it all the time. The scanner will also tell you why the cruise cut out the last time it was on. I could drive and record the electronics or I could install what we called the Co-Pilot and let the owner drive it and hit a button on it when the cruise cut out. Then he brings it back and I would download the info in the shop computer and I could see just what caused it to cut out. But have the codes checked first as it may have a code in it for the problem. Sometimes it would but not all the time. I replaced clocksprings and brakelite switches , cruise servo's and cruise switches along with even PCM's to fix cruise problems over the years. This sounds to an electrical problem and not vacuum related. Ron