Starting problem with 1998 Civic

Hi,

I have a Civic 1.4 1998 hatch, its done 140K miles and have never had any
problems with it engine wise. In the past few days when I have started the
car in the morning the engine kicks in with no problems but then instantly
sounds like its going to stall and the revs drop to about 500rpm, so I
give it some gas and after say 5-10 secs it stabilises at about 1500rpm
and eventually after a few mins drops to around 750rpm as normal. The
engine sounds healthy after that initial few seconds and never gives the
impression it would stall, in fact the performance seems to be good.
However, if I leave the car standing to go cold for 2hrs or more I have
problems starting it, to the point where I couldn't start it at all
yesterday. Based on my little amount of knowledge I gave it some TLC
replacing all the spark plugs and the air filter, also checked the
distributer cap, rotor arm, HT leads which were fine. The battery is only
6 months old and the starter motor sounds fine. I eventually got it going
and there was a strong smell of petrol so I don't think there is an issue
with the fuel supply.

Has anyone got any ideas? Could it be the auto choke, bad petrol/oil or
something like that?

Thanks very much