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

Please help.. Thank you