Terrecan engine misfire - mechanic baffled

Hi guys.

I drive a 2003 terrecan v6 petrol (it also runs on LPG).

For many months the car has had an engine misfire. The mechanics have replayed plugs (twice) coils (twice) and the leads but it is still happening.

Cost me a small fortune already. Mechanic said he doesn't know what would be causing it and had apologised and said 'we can't fix it'

Would really appreciate any help/suggestions.

The misfire occurs on both petrol and gas and is more noticeable when driving around town. The car 'skips' a beat.

Thanks everyone.

Darren