Upon acceleration the following lights illuminate once the engine reaches 3000-4000 RPM:

A message; "Engine Fault" appears on the dash screen. Once this happens, the engine will no longer rev beyond the 3000-4000 RPM range.
On a few occasions this has also resulted in a steering assist fault, where the power steering cuts out, and usually this also causes the engine to idle, and will not rev at all.
I have the following Ford Fiesta Model

Various mechanics have looked into this issue and all come up with different theories on what might be causing the problem:
- Faulty mass airflow sensor (this was replaced - no solution to faults)
- Faulty alternator
- Faulty ECU
- Faulty fuel pump
Does anyone have any experience of this issue, and what might be causing it?