how can i sort my vauxall vectras stalling problem?

I HAVE A 1999 YEAR,1800cc VAUXALL VECTRA THE PROBLEM I KEEP HAVING WITH MY CAR ,WHEN YOU START THE VECHICLE IT STARTS FINE AND RUNS WELL UNTIL THE VECHICLE S AT ITS ENGINE TEMPATURE ,ONCE ITS WARMED IT CONTINUES TO RUN WELL ,AS LONG AS YOUR NOT STOPPING AT JUNCTIONS OR TRAFFIC LIGHTS,ETC ANYWHERE THAT THE CAR IS SITTING IDLEING THE REV'S START TO DROP AND IT CUTS OFF,IV CHANGED FILTERS AND TRYED VARIOUS THINGS WITH NO JOY.