Hi Can anyone help?

I have a 1997 1 litre petrol hijet. It starts fine when cold but after warming up a bit 2miles odd it stalls and is a B***** to restart, usually it needs to be left for some time. It has been in the garage but they can't find anything physical and have said it must be to do with engine management? Is it when it gets hot the management system is not switching to turn off the heavy cold start fuel supply ie. a thermostat is bust or sumat?

Cheers!