What about the spark plug?
Have you put in a new one?
If it is getting gross fast, it may not be sparking well.
If it misses a beat every now and then, your plugs will foul up, and it will run poorly.
Clip the spark plug wires on both ends, and reterminate the connections at the plug cap, and boot. Clean the points, and put in a new plug.
Your symptoms sound just like mine did, I did that stuff, and now I'm cruising.