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Long winded but hopefully helpful (so what else is new? ;) )
Don't rule out the possibility of a fuel injector going bad. They feel identical to a spark miss. While the engine is running and missing unplug one injector at a time to find the culprit cylinder. This doesn't tell you if the injector or spark is bad, but you'll know which cylinder to concentrate on. Check the spark plug for that cylinder, if it's good continue to next paragraph. If not, replace it and retest.
From here I'm not familiar with the ignition you have, is it like the SVX with a coil on each plug or is there only two coils? The approach to determine the cause depends on which system is used. If COP (coil on plug) you can remove the coil but leave the wiring attached. Place an extra spark plug (leave the one in the engine) in the end of the coil and place the pair on a metal portion of the engine, away from any potential fire hazards, and start the car. Check for a big bright blue spark. If there is no spark you'll want to use a test lamp and make sure there is a negative pulse (12v system) and a constant power (also 12v.) If there is a lack of power or pulse you'll need to check the wiring and the computer/ignition module (whichever is applicable.) If pulse and power are present you've found a bad coil (assuming the spark plug is good.)
If you only have two coils it's much easier. First off, were there two cylinders missing or only one? If two cylinders are missing, are they co-joined at the same coil? If so you probably have a bad coil, wiring or computer. If not you probably have a bad plug, spark plug wire or injector. Just place an extra spark plug in the affected spark plug wire and rest the plug on a metal portion of the engine (away from any potential fire hazards), start the engine and watch for a bright blue spark at the plug tip. If there is no spark and only one cylinder on that coil is affected swap the spark plug wires for that coil. If the problem moves to the next cylinder the wire is bad, if not check the coil tower for a crack leaking to ground. If you have a spark and the plug is good you probably have a bad injector and will need a fuel pressure gauge to check further.
There is a small chance that there is a mechanical problem, but highly unlikely. The only thing that comes to mind that would be intermittent would be a valve hanging or a loose valve seat and both are almost unheard of these days. Plus they tend to be constant rather than intermittent.
Beav
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