To get the rotor gear out of the dist block, just take that snap ring off of the gear shaft.
Clean the contacts behind the block after the gear is out, and check the gear for any missing teeth.
Before you put it the gear back in, make sure you clean the block with alcohol, or some other solvent. You want that thing as clean as possible, to prevent the spark from tracking. That may be the problem you have.
Check the block over front and back for any carbon tracking. If that happens, the spark energy for one or more cylinders will go to groung at the mag case, instead of out the ignition wire.
Some good mag rebuilders have equipment to check the insulating properties of the blocks, before they send them out after rebuild.
Once that tracking starts, the block is junk.
I've been told that the 1200 sieres bendix mags make too much spark for their own good. Especially the 4cyl models. They will track across the block real easy, and will drive you nuts.
Make sure that the condensor mounting is very clean as well. I've seen a poor connection for grounding the condesor to the case cause problems there that will dive you crazy.
Make sure when you put the gear / block assembly back in, that you get the gear back to it's original position in relation to the rotor shaft gear.