i have a 2009 g37 coupe 130k miles

I recently starting have this issue where the car runs fine at all times except when i’m driving at less than about 40 mph and i release my foot from the gas and go to accelerate again. After releasing the gas the RPMs will drop to about 1k and when pressing the gas again it will simply maintain the RPMs there as if it didn’t care how much gas i’m giving it. Eventually when i give it enough gas it’ll start to stutter between 1k-1.5k and after a bit it’ll downshift and go back to normal acceleration. Sometimes ill also give it gas and the rpms will spike to almost 2k but then begin to drop back down even tho i haven’t let go of the gas.

the issue started happening after swapping my stock intake plenum for a ported one. since then i’ve put my stock one back on, checked for vacuum leaks, done the throttle relearn, and just about every single thing i could think of. the only thing i’ve been able to find out is that the issue stops happening if i disconnect the battery. but eventually it starts up again. the car doesn’t throw up any codes either and when i run through the gears in manual mode it runs perfectly. have no idea what to do next, would really appreciate any help.