i have 91z28 350tpi, i put a new GM crate engine in and it has about 80 miles on it and today the engine was missing while driving. it would fall on its ***. then it stalled at a light. well i took it home and now it starts but for only 30 seconds then it shuts off like i turned the key off. other times it will die slowly.while it is running the tach will jump all over the place real fast. the engine wont chrange its RPM but tach goes crazy. this leads me to belive it is electrical. it will run if i keep it revved but it misses bad. all sensors are new. dist is new, fuel pump is new fuel pressure is 40 at idle. check sig. for mods all mods have 80 mles on them the thing ran fine from the time it was frst fired up until today. The VSS is not hooked up since I have swapped in a t56. i do get a code for VSS but thats it. could this be my problem since i have speed density
here is what i have checked
volts are good they dont drop off before it stalls
fuel pressure does not drop off right before it stalls
oil pressure if fine and does not drop before it stalls
there is no codes
plugs are fine
coil is fine( i disconnected my MSD and put my stock one in. still dies)
all plug wires are on tight
connections at dist and coil are tight

HELP ME

 

I had a problem very simular to yours, And my battery cable was not connected fully. Make sure you tighten your cables to your Battery. Hope this was your problem........

 

also, double check the grounds at the back of the heads.