What is wrong with my Vectra? low/rough idle/stalling fault!?

I have a T reg/99 1.8/16v eco Vauxhall Vectra, generally it runs ok! but when I pull up in neutral, idle is rough for first couple of seconds, engine speed is down to around 500/600 RPM and sometimes it stalls as well!! Car will restart agin no problem.
1) It has only recently started doing this.
2) Car has been fully serviced since problem started.
3) Have had a diagnostic check done twice now, but no fault is recorded on the car's computer.
4) The fault is slowly getting worse.

If any one has an idea what is wrong, please let me know!
Thanks