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Check the brake switch function and adjustment either directly or using your scan tool.
Generally speaking, whether the TCC can be affected independently of cruise and brake lights depends on how the circuits are configured. On our squares, a brake switch slightly out of adjustment can cause the TCC to cycle erratically w/o affecting cruise or brake lights. The most common scenario is a switch bumped ever so slightly out of adjustment by inadvertently catching your toe under the brake pedal and lifting slightly.
those gm trans are controled by the ecm. your tps, brake switch, coolant temp, speed, MAF and gear dictate if the ecm will lock up the T/C. inside of the trans you have the valve body, faulty TCC,Bad o-rings, excess wear in TCC and a torque convertor soliniod that can cause the issue. sometimes cleaning the maf and resetting the computer via battery disconect can fix the problem or temperarly fix it. if memory serves me right its the 4t65 trans, the same trans we have in the impalas. some people never had one problem with them, others they turn into a nighmare.
Do you have access to a scan tool? One that can do live data streaming, if so, drive it while watching the torque converter clutch on/off data and confirm if that's actually what it is doing.
and dont let them talk you into a flush. its a waist of money and GM put out a TSB about not doing a flush and the only thing they say to flush is the transmission cooler when youre replacing the whole trans cause the old one crapped out