"Weak" cruise control
 

1992 Town & Country, 3.3L

My cruise control has the on/off button, the resume/accel button, and set/decel button. Recently, if I engage the cruise control shortly after starting the van, instead of maintaining speed, it will gradually lose speed, like it isn't trying enough, "weak" if you will. Say I set it at 40. A few seconds later it'll slow to 39, then 38, and continue getting slower and slower until I either hit the accel button or hit a steep incline, at which point it's like it realizes it's going too slow and has to compensate with engine speed to get up to the set speed. I let it drop to 30 to see how far it would keep going, and it didn't look like it was going to maintain 30 either. It's still doing some sort sort of speed control, because the decrease in speed is far more gradual than if you just normally remove your foot from the gas pedal.

This doesn't happen if I've already been driving for a while. At first I thought it might be a problem with a lower gear, but it also happened setting it at 60 on the highway shortly after leaving work.

Any ideas?

I'd look for a vacuum leak at the cruise unit under the hood.