Car cuts out , but why?

I have a G reg Vauxhall Nova. it has a manual choke. i can be driving for an hour and then for some reason when I pull up at a junction it just cuts out, everytime. it can't be that it's not ready for the choke to be in cos it's already warmed up by then. what can i do to fix this problem, it's driving me mad. I can't afford to take it to the garage andbe hugely overcharged!