How is your ignition timing? If it is a few degrees too advanced, it will try to fire before it is fully compressed and it will fight the final rise of the piston and cause interrupted cranking.
Using good fuel? I know you just tossed out the varnish, but did you replace it with regular or premium. Regular fuel can cause the same effect as above.
New battery?
Patrick Hayes Fremont CA MGNOC L-403 AMA EV, SPII, Centauro, Monza, SuperAlce