Caretaker

Hi, I've recently bought a 100 Series GXL with the 4.7 litre petrol motor. It has a LPG kit and runs great on gas but when I switch it back to ULP it runs " very" poor. I have found if I put a bottle of injector cleaner in the tank it runs OK for that tank but once I've run on LPG again it runs poorly again. I am wondering if anyone has experienced the same problem and if the are any suggestions for remedies. Cheers Kevin.