Nissan Micra engine problem. juddering

A bit of history of the car first. It's petrol 1 litre s. manual gear. I had it converted 2 LPG few yrs bak and it has been running in both petrol very well for last 4 yrs. now when I canary the car on petrol it Downey accelerates and engine light starts blinking. But as soon as you convert it to LPG it runs like normal. Even on motorway it goes upto 70-80mph on LPG but not more den 50-60mph on petrol and that takes really long as well. I had this sort of problem before so I had it checked and we replaced coils once. This time I had it checks. When mechanic removes the electrical wire from the cool pack on d 1,3 & 4th at pistons it judders which is normal but 2nd piston from the left it does not judders. I had all 4 coil packs replaced. Hav also replaced the air flow sensor. Have checked all the electrical wiring and everything seems to be fine. Neither mechanic not me are able to understand why the car judders on petrol and not on LPG. Please help!