I've had my GP since october and other that my Rad Fans not working i've not had any problem til now , my girlfriend drove the car yesterday and as she was leaving the college it died a few times but started back up, then this morning i drove the car and it did fine til after work but started spitting almost like it was cutting out , never did die on me but when you rev the motor up to about 2500 and let off you can hear a sound that sounds like backfiring in the exhaust. could this be a crankshaft sensor problem or fuel filter, or something more?