Weird, new problem arising..

[Bradydz]

I've got a 355 in my '64 GMC, and for the past few days, it's been having a new problem. I'll be driving just fine, no problem going a constant speed, accelerating, etc. But when I go to stop after a little while, I push the clutch in and start coasting to slow down, and all of a sudden, the engine stops. It kicks right back up when I turn the ignition to start it, and then continues to drive fine, as if nothing happened.
I'm not all that great at diagnosing problems, as I'm still learning to speak "auto". :P
Anyone have an idea of what could be happening? It's only done it 4 times in the last 4 days, and it seems to take around 15-20 minutes of driving, as it has done it near my work 3 times (which is about 10 miles), and then again tonight when I was almost home.

I will add that I do know my timing is likely off, as it likes to diesel from time to time.. Not sure if that could be a/the problem.

Thanks, guys!