cruise control will not hold speed

I have a 1989 Surburban 4x4 TH400 trans with 350TBI. The cruise control will engage and hold speed for 10-15 miles then slowly drop speed. If I tap the accelator the speed will resume to the desired speed and then drop after a few miles. All other cruise functions work ie. Acc/Dec, engage/disengage, on/off. In addition there is a slight pulsation while the cruise is engaged prior to the slow down. I have checked for possible vaccuum leaks and have cleaned and lubed the speedo cable. Any suggestions..

It's probably a bad cruise servo. The cruise control works on a vacuum system. If there is a bad seal or leak in the servo, it will "hold" the cruise for a bit and as it loses pressure, your speed drops. Happened twice in my Dodge. Check the vaccum hoses around the cruise unit and if that's not it, you might need a new cruise servo.

Does this happen all the time. I had an 88 full size Jimmy do that to me as well. I thought it was because i had over sized tires (33x12.50) and it was way worse if i was pushing a head wind as well......<shrugs>

I dunno...it seems odd to me that I had to have the servo replaced twice. It's not like we're talking about technology from outer space so we should have mastered it already. I came across a related thread (on "cruise", nothing about space aliens, that I know of...:-) ), as well if you do a search on "cruise". I'm having a problem installing an aftermarket cruise in my '03 2500 HD so I'm trying to gather some data.

Vacuum lea kgoing to the servo or a bad servo . Its easy to test. Apply vacuum to the servo and see if it holds. If not its bad