What kind of moped are you driving?
You're right. It sounds like the plug isn't the problem. Unless your flywheel or points are loose, I don't think the problem is timing, but it does seem like the ignition is what ails you.
I'll assume that the gas in your ped is newish and there isn't any water in your tank or carburetor bowl.
You could indeed be dealing with a bad coil or condensor. I'd be wary about buying a "universal" coil, though, as I've never heard of these and high tension coils are very precisely engineered for the engines they go into. Contrary to what some people may tell you, there is no easy way to test either the coil or the condensor without plugging them into an identical moped that works (and seeing if it stops working). Forget about testing resistance or conductivity! A bad coil can still appear good with these tests.
Look for loose wires or a faulty kill switch.
Have you tested your engine compression? It's possible that you have a heat-related compression loss. Try measuring the compression with a cold, and then hot engine to see if there is a difference.