I have 1996 bmw 316 with 280 000km full service history, recently gave it a full service but before and after there was cough like****** within the first 5 mins of driving when engine is cold at times when very cold it actually cuts out while driving and will struggle to start but eventualy does. Funny thing is it starts easily as a normal car would and drives well for about 2mins before it starts to*****. A computer diagnostic reports a faulty knock sensor.I dont know what this is. please help