Good compression? (need at least 120 PSI to run well) Sparkplug cap good(they fail due to the resistor opening up)Good fuel delivery?(Sparkplug wet?) Checked valve clearances?(a tight valve lets compression out). Tried changing the sparkplug connection at the coil and see if the problem moves?
I'd check the sparkplug cap and where the sparkplug wires/high tension leads go to the coil(s). Caps can and do fail with no warning, as they have resistors inside designed to reduce the RFI (heard on radio's and t.v.'s as a snap or pop when the plugs fire). Over time, heat and vibration cause them to quit working and replacing them is the only way to fix this problem(if this is it).
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