ive got a 1997 ford escort petrol,1300cc auto choke,car starts first time and choke kicks in fine but as the choke stops working the engine wont fire properly.it then clears after taking it down to a lower gear and revving it.as it warms it runs fine.the o2 sensor was changed when it got a new exhuast could there be a problem there. plugs HT leads are fine. thanks for any information that could help