misfire code p1316 -p01300

hi all
i have just had my p38 se 1996 to a lpg station and the guy said my car has a misfire on petrol and he can not set up the lpg because of this
The codes which have shown up are p1316 and p10300 when the car runs on petrol you can not feel any misfire but the code does show up
But when it runs on gas it does misfire and run rough can anyone help and say what i will need to do next i am not very practical when working on cars

First thing is to change the plugs and leads. my P38 did this every 20k miles and it was always the plugs and leads. Lpg will show up a missfire more than petrol.