306 TD stalling

Hi folks,
Grateful for your opinion on the following

Just recently I have noticed that the 306 is stalling on startup - it
runs for a couple of seconds then the revs suddenly rise to 1500 and
then drop and stall (with no input from me!). If I press the
accelerator for 20 seconds then its OK. The symptoms seem to only
happen when the I return to the car after it has been parked up for
half an hour - it never happens on first start from cold. I'm assuming
that its not the old classic of air in the fuel then, as it would do
this all the time, but it seems to be dependent on the temp of the
engine.

Only other things I can think of is its a bit smokey on startup
occasionally and if you leave the ignition pre heat on for too long
i.e. >5 seconds after the light has gone out then it wont start. You
have to time it right.

Thanks
Al