-For the Mechanics out there-

The Other Half has a -04reg Fiesta (ST Shape)
1600auto, that for no reason will cut out.
Only 3-4times in the past couple of months but i wouldn't know where to start, i only noticed it cut out when the engine was under load (stereo/lights/a-c) all being used, and it cut out on a roundabout. After pulling over and waiting 5 mins it fires up 1st time. its done bout 15k and has been well maintained

Any ideas?