Hi,
I have just had the exact same problem with my Focus tddi. When I accelerated over a certain amount the engine light would come on and all power would be lost until you turned the key again. My mechanic thought that it was the pedal too and got me a new one but it made no difference. It was then diagnosed by Ford as needing a new fuel pump. I managed to find one for £125 from Oldham Car Parts which my mechanic fitted and so far touch wood I have not had the problem again.
I hope that this helps.