Hello! I have a Chevrolet silverado 1500 -05 with M30 (4L30E) transmission. If I go on D at 50-55mph 1500rpm on a flat road it puts in overdrive as it should, but if it gets a little uphill or if I gas a little more then it jumps out and turns up to 2000rpm, and then jump in again seconds after.. Annoying! My friend who has simmular car as me has to "stamp" on the gas to make it shift down from lockup. Maybe the TCC solenoid that bad? Or a bunch of other things? Someone who may have more experience with this? The oil level is checked! Located just above the HOT mark at hot box, but hard to believe it would be the fault.
The brake light comes on as soon as I touch the brake pedal, you also hear that it sounds like it "clicks" like a relay. Riding at 50mph, steady gas and squeezed smooth on brake then it switch out of the lockup and increases 3-400rpm. If I released the brake it switched up again..

Apologize for my poor English..
hope you understand anyway