It sounds like your solenoid is failing closed. When it fails open, the engine is locked to the transmission. There is no transmission slip. With transmission unable to slip, the engine will stall when the car stops. When it fails closed, it won't lock up but it still can operate more or less normally, just worse gas mileage.
Things you can do quickly:
- check the wiring to the transmission, at the connector on the transmission, for a conductor broken inside the insulation. This is unlikely BUT can happen depending on various things and it is not a waste of time to check.
- if you can, swap in a known good PCM. This is time consuming. But one cause of a flaky TCC is the PCM is sending it faulty signals.
- check transmission fluid quality and levels; fluid should be *cherry red* and not smell burnt at all.
If any burnt aroma or if it is brownish at all, change it and consider changing to Dexron VI (never switch back if you do that - put a sticker on the radiator support). IF your transmission has never been open do not be surprised to find frass (little metal granules) in the pan and filter (if you cut it open).
- check https://www.facebook.com/ATSGcompany/posts/4t60e4t65etorque-converter-clutch-stuck-offcomplaintafter-overhaul-of-a-4t65e-th/346666345433103/
for more insights