Engine surging and shudder

[TransitRevolution]

I just bought a 2015 cargo Transit medium roof, 90,000 miles. Got on the interstate and experienced a surging problem. The rpms will drop 30-50 for two seconds then speed back up for two seconds, accompanied with misfire type shudder. Does not happen while coasting down a slope or climbing a slope. Happens constantly while on level or slight upward grade.

Any ideas? Cold weather? Needs high octane? Sensors or computer?

[TransitRevolution]

Additional observations: I drove 80+ miles using manual shifting and the symtoms happen not at all in gea s 1-2, much less in gears 3-5 and very significantly in 6th gear. I suspect the clutches are getting insufficient hydraulic pressure which will affect the higher gears more, with the shudder coming from clutch plates that are slipping.