2005 Ford Fiesta electrical fault

I am from the uk and I have a ford fiesta 1.25 firefly it is a 2005 model. I recently had an intermittent fault with the dials to which they would drop off and also the milage and petrol guage would disapear. But this would remain back lit. It would then start working straight away. Just recently it will not return and remains dead. It does not, whilst driving, effect the performance of effect anything else electrical. Once I stop and switch the engine off it will start again on turning the key. If I leave it for 10 seconds or so it will start and then die off. Once this happens I can not start it at all. Will not even try to start. It is just dead. I and my machanic have removed and replaced all the fuse and sometimes it will start up again and everything will work, for all of a few mins when driving. This happens with random fuses. Not one in perticular. My machanic says he has sent the ecu and cluster and something to do wih the ignition away to be tested but these came back ok! It has been over a month now and has spent a few days with an auto electrician to no avail. Any ideas?