Hey Scotty! I'm having a problem with my RPMs bouncing at idle in Park and Neutral (not in Drive or Reverse) in my 2004 Altima 2.5 with 276,000 miles. It also accelerates on its own slightly when you leave it in drive. It can reach up to 30-35mph on its own without the gas pedal. I thought it was the idle speed that was too high but later not because of the RPMs bouncing. I plugged in my scan tool since it had a code for P1148 & P1031. I tried changing the Air Fuel Ratio sensor that is on the exhaust manifold and it was not the issue since the problem still persisted with the bouncing RPMs. I have checked everything from spark plugs, coils, vacuum system for leaks, air filter, engine oil, and even the transmission fluid and all of them were fine.
Check your throttle body for excessive carbon or build up . Those cars were prone to oil burning I wouldn’t be surprised if it was keeping the throttle valve open slightly causing the concerns and the idle learn will have to be programmed with a scan tool
Check your front o2 sensor for damaged wires(the 1148), also the 1031 is your afr heating element.
I know you said that you checked the vacuum lines but did you try spraying brake kleen around the intake mounting surfaces, etc? If the rpms change, you've found a leak.
Also check your throttle plate & make sure it's clean.
There are far better things to use. I was spraying brake clean around under my hood, and some of the trim plastics started dissolving. The surfaces were ruined.
That's the acetone (why manufacturers who use polymer frames like Glock say not to use brake kleen as a cheap cleaner), thankfully there's both acetone free & non-chlorinated brake kleen now.
But yup, plenty of other options.