The same thing is happening with my transit at the moment it has been like it for about two months now, you are flying along when the glow plug light comes on and there is a sudden reduction in power usually when I'm overtaking a lorry on a hill on the motorway but all I do is switch the ignition off and on quickly while still travailing and it resets it self, this only happens a few time each journey I'm not sure what's causing it but I might find out the weekend if I can get it into the garage.