98 ram 5.2 help needed

Take a good hard look down the throttle body with butterflies open. If you see evidence of oil the plenum gasket 99% of the time is bad. The reason your smoothing out at higher rpm is that the vacuum is decreasing with the throttle open. At idle and just off the system is trying to compensate for the additional flow in to the intake system from the vacuum leak. Of course that's my opinion and how I would go about it. I had a 99 Durango 5.9 that started using oil misfiring etc. Didn't look too bad thru the TB but once I pulled it and sat it on my bench there was enough oil in it to make a royal mess dripping out of it. Others hopefully will chime in.