In the past I've had issues with the distributor bolt loosening, and the distributor moving on its own. Might want to check that
Also, does the cap look like the rotor has contacted it? If it seems like the rotor is loose, I would replace it. Mine have always been a very tight fit, with no doubt that it was tight.
If your dist., rotor, and cap all seem good, check your wires, and check your coil. Any of them could be faulty, or the coil could be on its way out. You may have a problem with the computer itself, but lets check the easy stuff first, shall we?