rough running disco V8 LPG

Hi, what could make my Disco1 3.9 V8i LPG run rough on both gas and petrol?

When i start it up and rev it a little it backfires once and then will run "ok" but not 100%

when i put the accelerator down half way it drives much better and goes up the revs quicker than if i put the accelerator down all the way to the floor, also when stationary if i put my foot to the floor it sounds rough and only revs to about 5000rpm whereas if i put the accelerator down half way the revs fly right up to 6000rpm and it sounds like it should.

please help

plugs n leads?

vacuum advance unit, static timing, lambda sensor (one of mine is wired to LPG so yours may be too) etc..