it is sometimes hard to start,some times it wont even start, after a while it does.some times when it barely stars,and i mean barely because i turn the key and it starts to crank slow and then it gains speed,but as i'm driving if i push on the gas too sudden it slopes and kills the engine if i don't let go on the gas.what system is more likely to be failing, fuel system, ignition system, starting system.....????