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?