it means that the glow plugs are faulty
I'm not sure why you have repeated this misinformation. Glow plugs do not operate at 50mph and do not cause engines to cut out.
Ford's glow plug light is the malfunction indicator lamp on many vehicles and identifies a fault through flashing once the engine is running.
The only other time the glow plug lamp usually operates is during ignition sequence and is to indicate the glowplug's are operating.
Several of the problems above sound like they have not really been solved, and misinformation doesn't help. Speed sensors can often present problems (located on the back of gearboxes sometimes) on petrol's also. Another problem likely that I haven't seen mentioned is residual petrol from an incorrect filling. These engines will often start with traces of petrol in the tank, when engine performance is more critical at higher RPM's the engine fails, and often when the vehicle is at normal operating temperature. Sometimes the ambient temperature effects seperation of the fuels in the tank presenting starting issues, sometimes the motion of the vehicle effects mixture which has its own symptoms.
A common problem I have seen is also the connection on the back of the fuel rail /pressure sensor, it is worth noting that a number of successful posts I have read confirm this.
Many of the other suggestions here are valid, its a shame that everyone doesn't post their successes on the forums, mostly you read of failure. Cam and crank sensors do fail, but they are made by companies like bosch and Marelli so its hardly worth blaming ford, worth a mention that Bosch make many of the fuel pumps aswel. The same company that make for GM, BMW etc etc.
I doubt that ford actually manufacture many of these failing components to be fair, and lets face it, its not like everyone is suffering from Big End Failure. I dont think ford can be laid entirely to blame here, they also suffer from software issues on ECU's, again the likes of Bosch make these. Perhaps worth visiting a bosch specialist electrician/service centre, they're advertised.
Having a proper diagnostic check using live data as well as or rather than historically stored codes is always better. There are often independent 'diagnosticians' that are very familiar with these issues particularly what the live data displays mean, money well spent, rarely more than a main dealers diagnosis, if you really don't know who to use, find out who your local snap on dealer is, and ask him who's bought the best tackle and knows how to use it, I'm not saying only snap on can solve it, I'm just thinking of an easy way for you to speak to someone who personally knows every garage and most mechanics in your local area, and is sure to know who's good at diagnostics, and who's shit.
Food for thought.