car dies

i have a polo 6n 1.4 8v and it starts first time all the time, runs fine. But every so often when it goes from high revs to low revs (if i put my clutch in) it sometimes cuts out and i have to bump start it again because it can happen at any speed

anyone had this problem before or know anything about it? I've recently serviced the car including all the filters, oil, plugs, leads and cambelt

any info will be a big help

thanks