Blake,
A couple of things come to mind. Are the plug wires Bougicord or Bosch? If they are US Borg Warner etc. toss 'em. Every set of US made lifetime guaranteed wires I have bought have ground out on the wheel wells etc. Usually the coil wire. Check your engine ground at the rear from the fire wall to the motor. Vibration can loosen stuff and will. If you can check with the previous owner, ask if the instrument cluster was ever removed. I chased a similar "voodoo pygmy" for two months only to find (as a last resort) that my speedo guy had left the wiring loose on the back of my cluster. Buy some dialectic grease and grease all the elect. connections, checking for tight fit and you can eliminate those posible causes. Have you checked the rotor and cap and air filter. I'm sure you have, I just mention the easy things that come to mind. Some times the intake hose will "wear" a hole on the bottom side away from view. Check Vacuum hoses while you're at it. Good luck!

Mark