Like that title states, I have a drunken CC.

Trying to set it any speed I can watch it drop at least 5mph.
Once it acted as if it were trying to downshift to regain speed, however, it didnt apply throttle. It simply downshifted. So just sat there at 3200rpm at 60 mph?!?!
Im about to go on a LONG trip and I will need proper functioning CC.

Thoughts on this?

2002 4.7 Quad

 

Check for a vacuum leak, damaged hose, etc., on the line to the cruise control servo. Also make sure there isn't excessive slack in the servo cable itself where it connects at the throttle body.

 

Location of servo?
Thanks

 

Passenger side fender, front. You should see a cable from the throttle body to the servo.

 

Awesome. Thank you sir.
Hopefully I can find something

 

Ok followed vacuum lines, looks damned near new.
Seems like there is a little slack in the cable till taught on the TB.
Can it be adjusted or is it replace only?
If it can be adjusted, how?

 

Some slack is normal. There is no adjustment really. There is a tab that fits in a hole on the mounting bracket at the throttle body.

Check this out though. It's from the FSM and seems to fit what you describe...

OVERSHOOT/UNDERSHOOT
If the vehicle operator repeatedly presses and releases the SET button with their foot off of the accelerator (referred to as a “lift foot set”), the vehicle may accelerate and exceed the desired set speed by up to 5 mph (8 km/h). It may also decelerate to less
than the desired set speed, before finally achieving the desired set speed.

The Speed Control System has an adaptive strategy that compensates for vehicle-to-vehicle variations in speed control cable lengths. When the speed control is set with the vehicle operators foot off of the accelerator pedal, the speed control thinks there is excessive speed control cable slack and adapts accordingly. If the “lift foot sets” are continually used, a speed control overshoot/undershoot condition will
develop.

To “unlearn” the overshoot/undershoot condition, the vehicle operator has to press and release the set button while maintaining the desired set speed using the accelerator pedal (not decelerating or accelerating), and then turning the cruise control switch to the OFF position (or press the CANCEL button if equipped) after waiting 10 seconds. This procedure must be performed approximately 10–15 times to
completely unlearn the overshoot/undershoot condition.

 

There's a one-way check valve in the vacuum supply that can cause that.

 

This is gonna certainly be worth a try! Thanks!

Please lmk where this is

 

OK so that was harder than expected. So far no dice.

Seems that the issue is not the same as mine does not return speed.
I can set it at 55, it will drop to 50 and hold. 75 and hold at 70, etc.
Wouldnt a vacuum leak continue to drop?