88 T-bird cruise control

1988 T-Bird Sport, 5.0 V-8

The cruise control recently started acting up and I am looking for the probable cause.

Symptoms: Once the cruise is set, it will hold the speed for less than a minute before it starts to fall off. It will drop as much as 10 MPH. A slight tap on the accelerator causes it to speed back up to the preset speed much like the "resume" button does after the cruise has been disengaged. Once it get there the falling off cycle starts anew.

The buttons obviously work. The vacuum dump valve appears to be okay since it will get back up to speed once the gas pedal is nudged.

Possible problem with the servo? I tried to find it (based upon the Mitchell manuals) but couldn't locate it.

Ideas? Solutions?

Thanks.

I have some cruise control stuff up under the upper back section of the front drivers fender on my '88 Mustang.