Octavia 1.8L engine running problems

I have a 1999 model octavia 1.8/20V 92kw engine. The engine vibrates as if one cylinder is not working. The problem happens only in the warm up phase after driving for about an hour. I smell petrol during cold start with some signs of backfiring/explosions in the exhaust pipe only in this cold start phase. No fault is registered in the ecu memory.

I have done series of test including air leaks-non found. Temp sender replaced, fuel pressure checks done,airmass sensor replaced with new.Learnt values erased with VAS5051, throttle re-adapted. Still no go.Injectors seals checked-ok.Powers supply to ecu checked-12V ok.

black Sooth found on spark plug tips and periodic poor acelerator response. I have high fuel consumption as well. Spark plugs changed but still no success.:confused:

I would be happy if any of you experience guys could give me some tips on this issue/or if any one has experienced such problems before.

Thanks.

TFSI