Jumping Tach

I have Acura RL 3.5, 1998. Every time I put the car in "P" the tachometer starts "jumping" around, between 1-2K rpms. I do not feel this jumping when the car is in "D" or "R". Fuel consumption has gone up slightly. After going to a few mechanics where they tried normal engine tuning, IAC motor, O2 sensor, finally, I have been suggested to go to the Dealer as the issue can be in the car computer. The closest dealer for me is 2 hours away and this is my only car.

My question is if I do not get this fixed, will it lead to a bigger problem?

 

I am assuming the engine is reving when the tach jumps (you should be able to hear it). Have any of the mechanics tried pulling codes from the computer to see if there's an error (is the check engine light on). I don't know the process on an RL but anything after 1996 is OBD II so any scanner should be able to pull the code.