My '95 1500 Aspencade has that problem. When I reach the speed limit of 65, I activate (or turn on the set button) the cruise. I still have to maintain that speed for approximately a mile, before the cruise will take over and hold it there.
From then on, the cruise works great, maintaining the speed up and down hills, and releases immediately when I touch the brakes or clutch.
I have changed the cruise filter, and while recently I had the tupperware all removed from the bike, I looked for the adjustment guage as described in the manual:
While looking through a square opening in the left hand fan shroud, locate a metal item with a V shaped notch. The throttle moves something up and down, and in the at rest position, the V and the throttle item should line up.
I spent two days, parttime, looking, and I cannot see the markings the manual describes. Finally giving up, as I was tired of my bike sitting still during riding season, I put it all back together today and went for a ride.
Illustrated Powersports DVD on maintainence of the bike, shows this adjustment, but they removed the entire left fan for better camera access. This is another days job, and nothing I wanted to tackle. Anyway, I have no indication this that adjustment is the problem.
Any ideas?