V8 with Multi point LPG. Year 2000.
I had a similar problem with mine, miss firing between 1500 and 2500 rpm.
I tried lambdas, MAF, plugs, HT leads, even HT coils but no change. In the end I disconnected the Lambdas and it ran like a dream.Has been OK for the past two and a half years. No change in MPG or power.
Earlier this year I changed the recommended, very expensive, plugs for standard V8 ones and the power increase was very noticeable.
I understand that the only reason for the recommended ones are in order to extend the service mileage requirements but it's very difficult to adjust the gap for LPG, in fact they say the gap is fixed and should not be changed. Try a set of standard ones as for the D1.