Suburban stumbles & stalls

I picked up a 1991 V2500 with the 350 TBI recently. I've got an annoying stumbling problem that's got me scratching my head.

The truck idles fine. It revs fine in park, although when I let off throttle it surges for a bit then winds back down (vacuum leak?).

When I put in drive, it runs OK if I feather the throttle but any more than 1/4 throttle and it stumbles and will die. I've checked TPS and MAP sensors for good operation. EGR valve seems to be in working condition. I just rebuilt the throttle body with a kit that included a new fuel pressure regulator diaphragm. Checked the timing and set to zero. Rotor + cap appear fairly new, as do wires and plugs. Problem happens regardless of whether hot or cold engine, and there are no SES codes.

The plugs had black soot on them and the truck did smell a little rich. Fuel injector connectors are somewhat melted (but still operational), exhaust muffler has some loose components inside which rattle and bang against muffler housing. These all seem to point to rich running...?

Not quite sure what to make of this, why does truck stumble and die in drive? The dude before me replaced the fuel pump (supposedly) and the fuel filter looks brand new, i suppose I should check he put it in the right direction of flow.

I've tried to find any vacuum leaks but no luck. Vacuum is around 15-20 inHg, and is almost steady at idle but has a very slight flutter.

Any ideas? Thanks

My home-made cigar smoker setup (which was hilarious and good smelling at same time) identified that the EGR valve has a leak. I'll replace it and post back with results.