Chevrolet Repair: Transmission Problem?, brake light switch, vacuum line

Question
I have a 1999 Suburban 1500 with the 5.7 motor.  The transmission was rebuilt about 15,000 miles ago.  When the truck goes into overdrive at about 45 to 60 MPH, about every 5 to 8 seconds there is a surge of 100 to 300 RPMs.  You can feel and hear the truck shifting back into a lower gear.  After 5 or 6 seconds it goes back into overdrive.  It continues this throughout the drive.  The service light does not come on. The truck has been into the shop numerous times for this problem and it has yet to be repaired. TP sensor was also replaced recently (someone told me it could possibly be the problem.  Any help you can give would be greatly appreciated.

Answer
Various sensors send input into the computer in order to enable the overdrive in the transmission.

It IS possible that a faulty and or mis-adjusted TPS could be the cause but it could also be a fault with the BRAKE LIGHT SWITCH under the dash or a problem in the overdrive solenoid within the transmission.

What mechanical work or ANY kind of work has been performed on the vehicle recently ?. As most problems that occur after a recent repair was made can be traced back to the area of where the repair was made as a vacuum line may have been knocked loose or a wire connection came loose, Look around in the area where work was performed..

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