Won,t start when cold

When cold my car starts first key and ticks over as it should do but when it gets warm it cuts out and won,t start until it cools down again. I,ve had it plugged in at the local garage and they are baffled as they could,nt find any errors on the diag machine. I,ve cleaned the egr valve checked for leaks into fuel inlet pipe etc but could not find anything wrong so I have run out of ideas what it could be as anyone had this problem before and know what the problem could be.