Polo Estate Start-Up Problems

[Zedexa]

Hi, I have a vexing problem with my S-Reg, 1999 Polo Estate. If anyone can get to the bottom of it I, my car-savvy dad, my brilliant VW mechanics etc. will be eternally grateful - it is really annoying us that we don't know what it could be.

Since I bought the Polo Estate 2 years ago, it's had a starting problem whereby sometimes it requires the accelerator to be pressed down in order to start. The majority of times it starts without the accelorator, but some days it won't. This seems to happen more on damp days, and more on cold days, but can equally happen on dry, warm days. There is no set pattern to it.

Once it starts in this way (with accelerator, having not started by just turning the key), it then requires about 30 secs of revving before the engine ticks over. Once the engine is warm it generally starts ok for the rest of the day after rest periods.

The problem is NOT to do with the following, which have been ruled out:

--Coolant sensor switch: this was replaced when I bought it and has been checked
--Computer system has been checked and rechecked and nothing shows up
--Stepper motor: this was replaced. This helped the problem in that it used to take 1-2 mins of revs for the car to tick over, and this was reduced to 30 secs after new stepper motor fitted.
--(As an aside, I also had a new ignition coil fitted 6 months ago when that failed, but this is nothing to do with this problem.)

Other than the start-up problem, it's a great car in every way.
Any advice gratefully received. Thanks.