95 Chevy K2500, Very Strange Timing issue

Hey folks, I'm really stumped on this problem. 350 TBI engine, high miles

Symptoms:

-cranks rough, but starts quickly (like initial timing is too high) sometimes, it will kick back instead of starting.

-in park, won't accelerate off idle with heavy throttle application, shudders violently. BUT, it will accelerate normally if running at 1500rpm or more.

-Runs extremely rough and then stalls with EST Bypass disconnected, no matter where you turn the distributor. Timing light drops out randomly.

-with EST bypass connected, the engine will run smoothly anywhere from

0-24degrees timing.

Tests:

-20in Vac steady at idle, 25in steady at 2500RPM

-IGN module tested OK, (swapped in spare just incase, no change)

-pickup coil measures in spec

-ign coil measures in spec

-cap and rotor, plug wires have 5000 miles on them look new.

-injectors have nice uniform pattern.

-plugs are a bit on the black side, just sooty not wet.

-MAP reads 1V at idle and 4.6V with key on engine off.

Why can't I set the base timing? Every time I disconnect the EST bypass she goes dodo.

Regards, Brent

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Brent
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