Since it just started doing it, I'd check for vacuum leaks and fuel delivery first. Make sure all the filters are clean and that the pump is squirting a strong stream when you crank it. I believe it should be around 4 or 5 psi and fill a plastic pint container in 20-30 seconds.
After that, you have to dig into the ignition and make sure your wires, coil, distributor components such as rotor, condenser are good. I'd check the timing even though you said it was running fine before. This is just in case something else caused it to go off.
Check the mechanical advance to make sure they move freely.
Check the vacuum advance by putting some negative pressure (sounds better than sucking
) on the hose to the carb. It should move easily and hold in place as long as you continue placing negative pressure.
Then after all this, you will probably have to do the deed and dig into the quadrajet... Not the easiest to take apart but it could have some gunk in iside.
Can't think of other things to check at this time.