I have GS300 2001. I have installed natural gas along side with the gasoline. When I increase the rpm above 3000, a strange constant kick appears as if someone is pushing and pulling from the back, or you may say kickbacks. I have recently changed the spark plugs and coils. And after several month passed with this problem the began to turn off all of a sudden. The engine can turn off eveb if I am driving 70 mph on a highway. It'll start dropping the rpm no matter if i am pushing on gas pedal. What kind of problem is this? If you have ever met this problem, please help me!

 

I actually found the solution of the problem. It was the MAF sensor failing all the time. Once I changed it, all the problem disappeared.