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!

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...