Well, it's gotta be an issue with spark or fuel, right?
Things to check would be:
- Sounds silly, but is the distributor bolted in tightly? (I forgot to snug the bolt tight on my 289 and the distributor walked)
- Coil (not likely, but it could be an issue)
- Faulty ignition switch (that will certainly cause "timing" issues!)
- Fuel Filter (I think you've replaced this already, but if not...)
- Fuel pump (obviously)
- Question - Is the fuel pump mechanical or electric? My buddy's 390 Mustang experienced a similar issue, which was traced to the cam/crank lobe that activated the fuel pump, and the lobe had become loose causing the new fuel pump not to function properly.
- Is the MAF clean/good?
I'm just putting some different ideas out there. I hope your mechanic can trace the cause of the issue before you have to throw too many new parts at it in the hopes of fixing it.