I find a CEL with no codes a bit weird. Tell them to run it until the light comes on, leave it running and check for both current and stored in IFI and ETC.6. (tranny codes can set the cel)
Resonance flaps, egr transducers and transfer pumps will cause intermitent driveability issues. Air flow sensors will cause a lean condition (eg, no power) as will fuel temp sensors. (which are in the shut off actuator)
There is also a prefilter in the tank. At this point I'd be curious enough to remove the sender and and a look inside the tank. (I had one full of ping pong balls once, drove us mad for weeks. Damn kids.
) You might have something wandering oer the prefilter/screen at the pick up starving the pump.
I find a plugged cat a bit hard to swallow, never seen a diesel kill one except for a few of the older traps on the 603's and calif. 617's. Hell a quick test is to run in straight pipes for a long enough time to see if it falls over dead again. (get earplugs, those things are loud as hell) Or weld in a O2 mount kit and hook a guage up. (the o2 kits are available from Summitt Racing)
There is a surge trick for some of them where you add a 250K ohm resistor to the a/f harness to force a slightly richer condidtion by way of dropping the egr duty cycle. I dont have the info right infront of me but, it would not fix your imediate issue right now anyway.
Joe