1990 F350 CCLB 4X4, 7.3 w Banks turbo, 5 Speed trans. I have owned the truck several years now, cruise control never worked since the day I bought it, and finally got it working a few days ago after cleaning the steering wheel switches. Took it for a test drive around the neighborhood, set it at 30mph, and used the set/accel to easily pull up to 45mph. Resume and coast also worked as expected. I figured all was well, until... ...road trip yesterday....I could set it at 65mph, but it slowly lose speed, about 1mph every mile or so. If I tapped, or held, the set/accel button, it would lose speed. I could feel the servo holding the pedal down when I set the speed, but it immediately started losing speed. So I said screw it, until the way home when there wasn't any traffic around and I could play with it more. I found it would hold 55mph fine, and even accelerate up small hills..but only to 55. If I set it at 65mph, it would again slowly loose speed until it got to 55, then hold steady. I'm ASSUMING it's a weak vacuum signal, or a tiny vacuum leak somewhere. But just wanted to make sure I wasn't going to be chasing a "problem" that's actually some kind of odd design intention, lol.