[17 sets of spark plugs in 9 months]

hello fellow rover lovers, I have a problem with my disco and I was wondering if anyone else has had and possably resolved a similar issue.

I have killed 17 sets of spark plugs in 9 months (a set aprox every 1000klms) they carbon up and stop producing spark, even after a clean they still no longer work, (plugs end up covered in a layer of black carbon)

I have tried NGK, DENSO, and BOSCH,(with gap of aprox 0.90mm with all)

I have replaced the coil, leads, distributer, distributer module, distributer cap, air filter and had the air/fuel mixture set by a land rover service department.. the coil reads 12.4 volts at the - and +, and I have done a compression test(incase of rings or valve leak) and they tested 175-180psi per cylinder, I have set timing and had it checked.

(although some of the above replacements probably werent nesessary in relevance to the issue.. im desperate)

1993 Land Rover Discovery (pre apr-93) 3.5L v8i(stock)215,000klms on clock, auto, consistantly serviced.

Any advice will be valued and appreciated, Thanks.