The 44D's cannuck mentioned did indeed have screw in slow jets... And they run awesome.
Perhaps the easiest and cheapest clean/inspect/rebuild I have ever had to do.
Drop main jetsize for altitude, and 45+mpg all day long.
But they don't compare to 750 carbs on a bored out '79 650.
Have since sold that bike(to pay medical bills) to a good friend, and he gets me off the line(gearing differences), but when I shift to second i've already shot past him.
Slick, have you doublechecked that all your electrical connections are tight?
I had a intermittent problem once, it was a connector for the timing plate.
it would short out(?) intermittently.
When it was wet, it wouldn't work and dry it was good some days, then 3 cyl action others.
In the end I sanded the terminals to remove the corrosion, and putting di-electric grease in the connector, and no problems from that again.
Possibly doing this will either fix your problem or stave off any electrical gremlins from popping up in the future from corrosion and water.
Your coil wires?
These are nearly 30 year old wires, you could have a crack in the wire causing a intermittent spark loss?
Trimming 1/4 off the ends and re-installing might be the solution.
Or new wires to be sure.
Have you tried swapping spark plugs around to see if the cold cylinder follows a bad spark plug?
You never know. I have once had a spark plug go bad and still look as if it is sparking correctly when resting on the motor. drove me nuts finding that one.
Good luck!
l8r