As said try another dealer, I do hope you are NOT being charged in light of the fact the car is NOT being fixed.
Ask them for a courtesy car, then don't hand it back until yours is sorted.
It's often a problem with intermitant faults, they are never there when you want them to be, it's even more annoying as it's usually something dead simple.
The clue is the fact that it does not flag up a fault code, this massively narrows down the range of things that can cause it, it almost certainly is a sensor, most have upper and lower limits, which if exceeded set up a fault code.
One that doesn't have limits on some is the CTS, coolant temp sensor, and if this is missing, a return signal to the ecu will go into LOS, limited operation strategy(ford speak for limp home mode), you can test this simply by unplugging it and driving the car, it should go into los and the cooling fan come on.
tabetha