03 E350 7.3 Power loss after 30 min of driving

[LukeFoley]

I have a 2003 E350 7.3L. After starting up, will run ok for about 20-30 minutes of driving, then will lose almost all power, shuddering. If I stop for 10-20 minutes, then restart, will run sorta ok for a little, then same problem again. Has been to mechanic 4 times, and they are clueless.

It really feels like it is fuel-starved, like it's simply not getting enough fuel.

-No check engine light
-Checked fuel tank for sediment/blockage, is fine
-replaced fuel filter
-replaced 2 fuel injectors that they said weren't functioning properly
-has aftermarket fuel pump

This seriously sprang up out of nowhere...I was out of town for 2 weeks, and it drove fine before that, then came home and have had problems ever since.
Please help, my mechanic has no idea (or maybe just wants to nickle and dime me).

NOTE:
Not sure if it is relevent, but I also have a second tank which uses SVO (straight vegetable oil). Last time I drove it, I tried switching to the SVO tank, and it pretty much ran fine...then back to the diesel tank and lousy again. Tried switching twice more and same results...so maybe something in the diesel line?

Also, ever since this problem started, has been shifting kinda weird...not sure if a related problem.