2009 g37 sedan. Car will start shifting erratically as if it does not know what gear it should be in which causes it to go into limp mode about 30 seconds later with no CEL present, but here is the weird thing. When the car goes into limp mode it will shift, but extremely violently as if someone is rear ending my car every time it shifts. To fix it i just disconnect the battery and reconnect it. This happens usually bout once every other month. i also notice that when i am driving down the highway, steady on the throttle to maintain speed, the car will feel as if it brakes on its own or like someone pulling back on the car to prevent it from going. This happens for like a second then it resumes a normal feel.

Does anyone know what might be causing this.

As a side note: when i take the car up the mountain (lots of switchbacks and steep terrain) EVERY SINGLE TIME it does this which i think is odd

 

Still learning how to diagnose, but IllI take a whack. Senior members, please correct me. Ruthlessly.

Limp with no CELs and a sticky trans, especially with the switchback behavior makes me think (1) a disintegrating clutch pack, or ow/very old/dirty/burnt fluid. &/or (2) transfer case/torque converter is low or leaking (known issue) / the fluid is bad.

How many miles? Are the fluids changed on-schedule?
When its behaving, are shifts getting slower, harder or clunkier? Do downshifts into the powerband hesitate? (ex. From 4th at ~2500rpm to 3rd =~5000rpm).
Iirc, you can "test" the clutch pack integrity by flooring it up a slight incline in 6th (e.g. ~1100rpm WOT). Does power drop or does it lug or hesitate? Feel like its trying to put the power down but is slipping?

​​​​​​Since you can reset it with a battery disconnect, maybe the TCM forgets that the trans is out of its parameter range, and starts relearning as if all is normal. No clue if it would throw a ODB2 code...

Limp mode is no-joke. I'd eat the $ for a dealer diagnosis, then take it to your favorite mechanic.