1250 troubleshooting
 

I've noticed a few things about the 1250's supposed "misfire".
1. It still sounds like it has ignition. When the "Misfire" happens, it still pops like if the plug were still igniting the mixture.
2. It starts beautifully, most sources say your engine will hard start when it has a misfire.
3. It only happens after the engine has warmed up. It gets really bad at higher RPMs.
4. Happens when hitting bumps or when the engine is not under load.

Possible causes(?)
Dirty air filter? The air filter is filthy.
Exhaust valve not seating?
Bad coil or points? Seems like the coil is newer though.
Bad spark plug or wrong plug? I don't know, haven't checked it yet.

Sounds like a bad wire, or wiring connection.

I had a similar issue on my 1450, and it turned out to be a loose connection on the Condenser where it mounted to the engine.