Power surging

[GCrowell]

This is my first post in the 2nd gen section. I usually hang around the 3rd gen group. This question is on behalf of my nephew.
He has a '96 3500 with auto tranny and about 240k on the clock. He bought the truck second hand and has had it about a year now. Problem developed recently while towing a medium sized 5th wheel trailer. He has a heavy foot and probably tows above 65 most of the time.
Last time he was on his way home (FL) from MN with trailer in tow he experienced an intermitant drop and then a recovery in RPM and power. He barely made it home by babying it along. When the trouble occurs, he can not get it above about 50 mph. During the events, it loses enough power to slow to below about 40mph. He has changed the fuel filter twice now with no noticable improvement. He took it to the Dodge shop and $75 later they could find nothing out of line; fuel pressures in the shop are 'normal' (don't have the numbers). It will not do it every time he drives it and it will also do the surging when not towing but seems to be worse when towing. He has no gages.
He is a pretty handy wrench but needs some ideas on where to look.

Any suggestions would be greatly appreciated.

Thanks,
George