AUII Fairmont troubles.

[Crash Lander]

Hi guys.
My old man has an AUII Fairmont, and has had heaps of trouble with it. It's been converted to LPG.
Current issues are:

Petrol mileage. Currently getting 15-16 L/100km. I've told him to change the O2 Sensor. However, I'm not certain this is all of the problem. He said when he started it the other day, the revs just kept rising by themselves, and wouldn't stop. He had to turn off the key to stop it. Re-started the engine straight away with no problems.
Also, the other day, he was driving along with the cruise control on, and started to run out of LPG, so he switched over to petrol without disabling the cruise control. When it switched to petrol, the car took off, well past the set cruise speed. He said it took off faster than he could ever make it take off. He's always said the car is pretty gutless. He said if he'd planted his foot, the car would not have taken off as fast as it did when this happened.
One mechanic has said it sounds like an engine management system problem. Does this sound right? This same mevhanic told my dad he didn't believe that I get 750-800km regularly out of a tank of petrol in my AUI Forte. Ideas?