Misfires to nearly stalling?

My beautiful Series 1 Disco 3.9 v8 Petrol is misbehaving! It's got lumpy tickeover, misfires to nearly stalling (clears at high revs), and only changing the spark plugs fixes the problem. Trouble is, I have to change plugs every 2 months! It labours the battery badly on starting if its been standing for a couple of days. Any ideas?!

Forgot to say, when changing the spark plugs, they're clogged with black carbon

Sounds like you have a fuel mixture problem, are you running with carbs or injectors? if carbs, check the fuel air mixture and jets for clogging.
Plugs should be a nice biscuit colour if all is ok.

Find a garage who understand the way oxygen sensors work and get them checked - sounds very much to me like they are failing to switch rich so the Ecu is widening the injector pulse width and getting no repsonse,so it enriches as much as it can and is fouling the plugs.At high speeds and loads the ecu does not use info from the oxy sensors - it just uses a set fuel map,hence clearing itself.

Thanks very much for your replies. It's injectors not carbs. We did replace the oxygen sensors a few months ago, which fixed the problem immediately, but problem started again 2 months later, so the problem existed with both sets of oxygen sensors? Thanks for your continued help!

Check that the oxygen sensors are being heated,they receive a 12v supply from the centre blade of the fuel pump relay.They are also earthed,cant remeber the wire colours on the sensors,but it should be easy enough to work out.Sorry to be brief,but a rather nice smelling roast dinner is on the table !

my j plate v8 had very similar problem, was an air leak on the valley gasket and the amp on the distributor was fooked