I know exactly what you're talking about...it's caused by a transmission lock solenoid (or something like that?), check with the dealer for the replacement http://gmpartsonline.net
I had to replace mine on my Silverado and it went back to normal...my gas mileage also got much better, oddly enough.
I don't see what year your vehicle is but the housing is cheap here --->
http://www.gmpartsonline.net/parts/2009/Chevrolet/Tahoe/LS?siteid=215055&diagram=GC07715&diagramCallOut=3
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