Can't say for sure what would cause that, I'd say you have an issue going on in your ignition system or your light isn't good, misfire could be the cause.

Why are you retarding the timing to 0 though? The engine will likely not run best with 0 degrees of initial timing, and also if it was on high idle the mechanical advance was likely already coming in (and vacuum if it was hooked up still). The timing shouldn't have moved from the last time it was running.

I would advance the timing and try to stabilize the running conditions first. And let it warm up and get down to idle speed.

For what its worth, stock 2 barrel small blocks even seem to like at least 10 degrees of initial timing. Just make sure once you set it you don't have too much total, it should be around 34ish without the vacuum advance hooked up once the timing is all in. Stock Chrysler electronic ignition?