Also, you said you gave it a tune up, but didn't mention the distributor cap and rotor. If those weren't changed, then that would be a great place to start checking around. Sometimes the chalky-white upper crust can be lightly cleaned off with sandpaper, but if it's gouged, burnt, melted, or the cap is coated with a rusty orange dust, the gap is too wide on the rotor/cap, and both parts are too far gone to be saved.

Wow, that was one hell off a run on sentence. I'm going to assume you changed it already.... what did you set the plug gaps to?