Sparkplugs???????

What brand of plugs? Bosch platinums?

Just some ideas, probably wrong but I've a lot of high voltage hostile environment experience in military electronics (and they go wrong all the time), so for what it's worth...... Are they resistor plugs? Measure with an ohmmeter from the connector to the tip, should be the same as the good plugs within 20% ish. Could be going high in resistance due to too high a voltage & current, the only causes I can think of are a very wide gap in the distributor cap/rotor arm for that HT lead (I don't know if an R30 has a distributor) and perhaps an open circuit HT lead where the spark is jumping an internal gap so you don't notice anything wrong you just get a higher voltage developed before the plug will fire. Another cause of resistor failure is engine vibration, this would have to be worse just on that cylinder though, so it would have to be a high frequency resonant mode local to that part of the engine or the other plugs would suffer as well, perhaps a worn bearing on that cylinder or just a loose bolt? A length of tubing used as a stethoscope should be able to tell you if there's extra noises in that cylinder area. A different brand of plug may help (I like NGK in Japanese motors).

anything does it

Champi>

I don't think it's the sparkplug. It may be a bad cable or connector. The plug is fouling. Also check your grounding wires from the engine to the chassis.

Try the right NGK's and then when the plug "dies", swap it into another hole instead of replacing and see what happens. Might be a clue.

The plug is not fowling........... Looks to be running in perfect condition