Enigmatic Timing? MAF? Electrical? Problem

I'm wondering if anybody has a clue about what's going on with my 93 4x2 Truck.

One of the truck's previous owners ran it off the road, through a barbed-wire fence and into a plowed field. That may or may not have something to do with my problem. I offer it as background.

I've recently replaced the timing chain, chain guides and chain cover. The chain had eaten the guides and gone through the cover to open a hole to the water pump.

I've replaced the rotor and cap, plugs and wires. (Which seemed to help a little)

MY PROBLEM: the motor skips--when it first starts skipping, it occurs at mid-revs under a load. It seems to occur MORE often in 3rd gear than the others, though that may be because of the mid-rev range of the phenomenon. If the truck was just skipping like that, I could get over it, but eventually, the motor will arbitrarily just stop. I could be driving it or I could have just started it. When I try to restart it, I can only keep the motor running by pumping the gas. The exhaust smells really rich, and at some point I can let it stall out and it'll restart almost normally.

The other day it happened with no warning--one minute I'm driving up a hill, the next, the engine has died. Pump the gas pedal to keep it 'running.' Rich fumes. I had an idea. I checked the air filter. It wasn't dirty enough to cause a problem, but I tried starting the truck with the air box open and the filter out. No change.

Now the hopeful thing--When I replaced the filter and closed up the air box, the motor started and ran famously.

HELP!!