Aloha,
I have a 1991 E-350 7.3 Diesel. I've owned this vehicle for nearly 7 years. For the last 6-8 months the engine has been surging from 35-55 mph. It's like it's gasping for more air or fuel. She will also randomly slip out of gear when in the higher gear only... I'd say around 45 mph+

Recently I have replaced the ECM, Fuel Pump Injector (what a pain in the *** and expensive), the Throttle Position Sensor (4 times - can't get a read on this)...

I did hit a rock on the transmission case AFTER this surging thing started which smashed the solenoid. I replaced that and the slipping in lower gears went away.

The positives from all the recent repairs, less to almost no smoke, starts up super fast even when cold, hard shifting is gone, feels all around faster... BUT still has the surging and higher end transmission slip.
I'm starting to think this may be a transmission thing.... it's a rebuild transmission from 7 years ago. I live in Hawaii and am constantly driving up steep hills... I use fuel injection cleaner... I'm extremely discouraged as I love this van. It's a huge part of my life and an expression of who I am. It has all the surfboards in it, a bed, bench seats from my dads van when I was a kid... If anyone has an idea of what could be causing this surging please reach out.

Side note - it does not surge at idle or in lower gears.

Mahalo,
Kathryn