1997 transit lpg

[poundland1]

i've had this van for about 10 years and had the occasional problem,but this time !!!.The van starts up ok when cold then when the engine starts to warm up it coughs and splutters then sometimes ok then problems again,i have noticed that the change over from petrol to gas takes a lot longer than usual and approaching junctions the engine is racing.The other morning -4 degrees heavy frost, start the van ok but within about 1 min the electric fan kicked in for the first time ever.I took the van to a ford dealer left the van switched to petrol only but they found nothing.They said it must be a gas problem.HELP.