What is happening to the cap? Is it burning up or it there actually the rotor touching and getting chewed up or something else?
Is the cap and rotor on there now new and ok? Has the distributor ever been out recently or moved? Have you double-checked the firing order? If it is still misfiring do a cylinder balance test and tell me what cylinders are missing.
First of all you will need a 12-volt test light and about 8-12 inches of neoprene vacuum hose. The vacuum hose will conduct electricity cause it is carbon based, use an ohmmeter if you are not sure and see if the vacuum hose you have will conduct.Now, cut off the same # XXXXX cylinders you have in small equal lengths of the vacuum hose. 2-3 inches will do fine. The small diameter kind like to carburetors, etc will work as long as they fit over the distributor cap tower connection.Now, mark all your spark plug wires at the cap or coil pack and remove them all. Put those short pieces of vacuum hose on the distributor cap or coil pack connections and shove the other end of the vacuum hose into the spark plug wire boot until it makes a good connection.Now connect your 12-volt test ground clip to a ground and start the vehicle. With the engine running touch the 12-volt test light to each of the vacuum hose 1 at a time and listen for the cylinder to short out and die and drop in r.p.m. They should all be about equal. If 1 or a few don’t drop or do anything than you have your dead cylinder there.