When in cruise control at 60-70 mph, the engine will suddenly rev. It's like the cruise on a stick shift and you bump it out of gear without using the clutch and the engine red-lines thinking it's still accelerating. I have to quickly tap the brake to disengage the cruise control. Usually, it occurs on an uphill grade when it attempts to downshift. But it has occurred on straight and flat road as well.

This is a new/corrected observation. It happened again this weekend, only I was slower to disengage the cruise control. It appears is is downshifting to to low a gear. It is revving quite high but only as high as that gear and speed will allow.

Any thoughts?

This is a class C RV, 1994, E-350 chassis.