Fiesta 2007 losing power, no errors when tested, many parts already changed

I'm wondering if anyone can help me 

I have a 2007 Ford Fiesta diesel and everything was fine for the first year

Then occasionally it would stutter when driving and after a few months the engine light would come on and off at random and then it would lose power 

I could be doing 70mph and then within 5-10 seconds I'd be at 20mph, foot flat down on the gas with no acceleration, then a few seconds later the light goes off and it's back to normal, next time the light may come on "says: engine system fault" as soon as the ignition (not engine is on) then I can have a week of no errors then a couple days of limping around again

After taking it to the garage, they said it was the EGR valve feeezing, booked it in a garage and 3 days before it wouldn't start, I had to jump start it each time (even though there was still some power to lights etc)

EGR valve changed, starts normally now but still going into limp mode

took to a second garage to see what they would say, they said a shaft for the EGR valve was faulty ( sorry can't remember the name)

changed that, still goes into limp mode at random, next I go back and an error for "mass flow air metre" not responding comes up

that gets changed 

still going into limp mode

i now go back and now no errors are appearing (even though the dash board light which is an orange {!} is flashing and says "engine system fault"

Tried taking it to ford dealer and they said I need to change the EGR valve, nope said its just been changed so left

Original garage that did the work say they can't find anything wrong and the computer shows no errors

recently lost acceleration power 100% and called for roadside assistance and their computer also found no errors, then after about 20mins the light went off while he was there and worked fine for a few hours 

at a loss now, wonder if anyone can offer any insight as to what's happening 

service, fuel injector fluid etc have all been tried too