RPMs drop when stopping

[rares]

I have this problem for 4 years now and I have tried everything , by myself(cleaning) by the dealer w their expertise and end up getting zero results.When engine cold or the shift in sport the idle rpm doesn drop bellow at stop (NO REAR END FELLING FROM TRANSM).I'm affraid that keep driving like this I will end up replacing the transfer case again.To have this problem fixed I replaced throttle body,trasm software update,all vaccum + hoses replaced,fuel filter,trasm filter=oil changed ....and still complaining.Maybe there are some sensors that send or doesnt send the right info to ECU and I dont know about it but the problem seems obvios and easy to fix .If you keep the car runing,warm engine ,shift in P and give a little gas up to 2-3000 rpm and then let come back to idle speed you will notice that idle goes under500rpm abruptly(not normal) and then comes back to 9-800rpm(normal).