Highway Speed High RPM's
 

Hey Guys, been having an odd issue with my truck and wanted to see if anyone maybe had some suggestions.

1990 F250, 351W, C6 tranny. Last run I did with the truck in November before winter, I was finding when I got to roughly 55 mph where the motor would normally settle down into it's "highway mode", the engine would stay in high RPM's and sound like a jet taking off.

Little background, motor was pulled at the end of 2017 for a refresh. Seals, manifolds, timing chain, tune up, EGR tube, etc. Hadn't had an issue other than the water pump started weeping from the seal for whatever reason at the end of 2018.

September 2019 the power steering pump and lines were replaced along with the water pump again. A month and a half after that is when this issue started. Don't know if there's any relation.

I put a different vacuum modulator in the transmission to make sure it was shifting. It clearly is and the fluid looks good. I replaced the fan clutch, belt tensioner, and belt is case there was slippage. No change.

There's been so much gone through and replaced on this truck the last couple years I'm not sure what it could be. The truck seems to idle fine, runs great at low speeds, doesn't surge, backfire, or stall.

Thanks