LPG problems 2006 Transit

[David W LPG]

Need to find some info on 2006 LPG Transit. Runs on LPG then switches back to petrol. Reset (ign off) and restart. Runs again but for a shorter time (maybe a mile). Gradually gets worse. Cool down and it will run for 4 miles on gas before it starts playing up. AAggh!
Regards David