LPG Vauxhall combo problems

[humeally]

I've got a 53 dual fuel combo van. Sometimes after its been running for a while the lpg switch light comes on and stays on, it stays running on gas, anyone know what this means. I also only seem to get 25 to a gallon of gas. When it is ticking over on gas the revs kind of jump up and down a bit. Changed the spark plugs tonight but it still ticks over funny. The exhaust has a slight blow on it, dont know if that would do anything

Any help appreciated

Ally