TheAutomaTom

Disaster strikes:
For 6 months, I have enjoyed driving a 1991 Scottsdale, 309k miles, 5.0L 305 V8. A couple weeks ago, I pulled into a parking space and it stalled out. It never started again, and had to be towed.

Present condition:
Using my timing light, I can set the distributor to 20' BTDC and it starts (but idles very poor). While then running, I can turn set the timing to TDC, and it sounds great. But... if I turn it off... it won't start, again!

What I did in between:
Almost every sensor and doo-dad on this truck has been replaced in the last 6 months: Distributor, cap, rotor, ignition module, coil, wires, plugs, IAC, EGR valve, oil pressure sending unit.
Gas is clearly visible at the TBI's injectors.
Test light fires on plugs.
Compression test showed 130psi as min on all cylinders.
Now, I assumed the timing chain was suspect, so I replaced it.
I set the #1 on TDC and observed the old sprockets' marks both lined up at 12 o'clock. There seemed to be slop in the chain, but the brand new set didn't fit much tighter. It is a cheap, stock Autozone product. You can see the old cam sprocket had a GM logo, so I assume that means it's all very high mileage.
I have verified the distributor's rotor is pointing at the #1 wire as expected at TDC.

WTF? What would prevent the engine from starting within an expected timing spec, even when it can get running waaaay off, and then tuned to idle well? Again... this truck ran great, unti lI pulled into a parking spot and it stalled out before I turned the key off. I hadn't even popped the hood in weeks before that. No leaks or oil burning, just a truck load of frustration! Thanks for any advice or inspiration.