One of your injectors might be sticking, Put a timing light on each plug wire it will flash consistantly if the plug is firing, otherwise it'll be off or fire inconsistently. If you find a missing cylinder, that one will most probably have the stuck injector. The plug sometimes won't fire if the fuel mixture is too rich or too lean.
MSD uses all kinds of systems, if you have a vac advance ont he side of your distributor, it's using that to advance, and if you take off the cap and look you will either see weights or not. If no weights exist, it is adjusting advance according to RPM. The manufacturer of the engine will have a spec for total advance at XXXX RPM.
The timing could be off wildly at idle before it will statr to miss, you could be off as much as 20degrees and have little effect on firing, but it will idle rougher if it's off by more then 4-6 degrees. Put the timing light on the crank and see if it's in spec. It's missing on idle, so only worry about idle for now.