car keeps stalling

Hi, im new to this site and thought id give it a go as i have a problem with my 306 its a 1998 1.4 meridian and recently it has started pulling on acceleration (only occasionally, not constantly) and if i am sat in traffic after 1 or 2 mins it just cuts out! It starts fine again but as im pulling away it starts pulling.

I put it in my local garage and they told me that they could not see what the problem was, however they replaced the crank sensor??? and cleaned leads to the battery, he took it for a test drive and he left it running for 30mins and it did not stall, so i thought the problem was solved! clearly its not! I do not really wan to put it back in the garage if i do not have to, does anyone have any ideas as to what it maybe??

Thanks