Got the car back from the mechanic. He said the issue was that the LPG system splices into the factory oxygen sensor wiring. However, someone in the past has removed the stepper motor for the LPG system, which means it now runs open loop, and doesnt need any o2 input. The splice connection was twisted and taped, so it would sometimes send dodgy signals interfering with o2 readings to the ecu when running on petrol.. thats why it was running good for a few weeks, until it wasnt. He's now disconnected the LPG wiring to the o2 sensor circuit and terminated it, so it wont happen again and the car is running good again... for now.. haha.