I recon its the CPS. This has happened to me on my Fiesta RST. The one in the FRST was know to cause owners some stress occaionaly! They can be bought from Europarts or EBAY.

Do you know anyone with a EFI CVH engine? f so borrow there one for 5 mins. It is a very easy job to swap out!

If you have an osiloscope or know some one that has you can check out the saw toth wave output from the hall efect transducer.