I've had my Fiesta from 3 years old (ex-mobility) and it only has around 72 thousand miles on the clock.
For several months the revs have been periodically dropping, whether I am at a constant speed or accelerating and irrespective of speed. Initially the revs were dropping from 1500 to nil then straight back up, now they are dropping by a couple of hundred revs at a time (so at least i'm not nearly thrown through the windscreen now).
Today on the way to work my car lost revs once, after after 5 minutes, during an 11 mile/20 minute journey. It didn't lose revs at all on the 40 minute return journey (20 minute traffic jam). A couple of days ago it was like a kangaroo on the way to work but perfect coming home.
Diagnostics show nothing each time and the garage hasn't been able to find a fault.
Typically it never does it when the garage has it, even though over the past few months they have had it for several weeks.
It has had a new fuel filter, two new coil packs (when replaced the first time it stopped doing it for a while so we weren't sure if the first replacement was faulty), new plugs and leads and a new air intake thing (can't remember what it is called) and another new part that sits close to that.
Does anyone have any idea what is happening to it and how it can be rectified?

 

What model fiesta is it? It could be an idle control valve, take it off and give it a good clean with something like wd40 and see if that makes it any better.

 

Sounds like the gearbox is on its way out... My Mrs's old 1.25 auto mk4 done this and then started stalling at traffic lights etc, i old changed the gearbox and it helped for a few thousand then it started again!