1968 Ford Fairlane running concern

Hi, I am in uk I have a resh rebuilt 302 30over 289 heads cleaned and rebuilt weind street wariour intake and a holly 600 vac sec carb car has been running lovely was overfueling so I sorted mixture and had a tune up guy come everything spot on goes great down the roard but stall at juntions alough it idils fine on start up and when left in park very confusing any idea?