My cruise control went out on my '88 R10 Suburban the other day. It had been working fine until I got on the interstate and got her up to 70 mph. It started having problems holding the speed but I thought it was because I was going up a hill until it started happening on the level parts too. It finally quit on me when I hit the brake to let a semi in. It has not worked since.
I'm not sure where to start looking. If it is a matter of replacing parts, I should be able to do that myself as long as they are accessible.
If I had to take an educated guess, I would start looking at the transducer. That's the piece the throttle cable goes to. Check the wiring connections and vacuum lines at the transducer and verify that you have good vacuum going to it.. Without a diagnostic trouble chart, it's going to be hard to check for correct voltages at the various components. Probably not the brake switch because it wasn't a sudden failure. Could be the multifunction lever, but over the years, I found that the transducer was usually the problem.. Might be hard to find new parts for a system that old..