1994 960 & LPG problems

Hi I have a 1994 960 3 litre estate fitted with LPG back in sept 2006. It ran fine till a couple of months ago and then backfired several times etc. Now The performance of the car is sluggish in both Petrol & Gas. It revs up and down when cold, and just seems "wheezy", especially when cold. The system I have is a millenium electronic feedback system. Some one has suggested that my lamba sensor is faulty Is there anyone who can help or provide some advice of some one I can go to who is an expert in this field? Thanks Terry

Thanks 

Hi Thanks for the lead, I have picked their contact details and will contact them. Thanks Terry tel07704 678 398