When the problem occurs, you need to check for spark at the plugs. I know you said you had spark at the coil wire, that's good, but you need to check for spark at the plugs. When the problem occurs, if you have spark at the coil wire and no spark at the plugs, then the problem is either the dist. cap or rotor. If you have spark at the plugs then you can move on to the fuel system.
I did'nt think to ask were there any codes?