OK, if the plug caps are not arcing it would seem to be definitely temp related but I would do two easy checks.
1.) Run a compression test on each cylinder. Throttle fully open, just disconnect the coils, (This will give a service warning but that is normal.) and crank it over for five seconds. DO NOT leave the other coil connected! You should get a figure of around 175PSI both sides. If one side or the other is much higher than the other it is likely to be....
2.) Check the cam timing.