My cruise is also notoriously crappy.
I think it has to do with the gutless engine and oatmeal in a bucket that they pass off as an automatic transmission.
I always set the cruise about 5 mph higher than needed because it always drops some speed initially. I don't even bother with the accel button because it will actually slow the thing down unless I push it and hold it for about 5 seconds, at which point it will down shift and lunch the thing forward.
It will not maintain speed even on a slight incline. Once it drops by about 10mph the thing kicks down and the engine revs almost to redline causing it to over shoot the set-point by about 10 mph.
The only thing that seems to work for me is to hit the O/D off button (on the shifter) before a hill to make it down shift early and the extra RPM tends to help it maintain it's set speed.