I had exactly the same in my Brera 2.4JTDM. I have a 40 min journey, 25 mins on 60mph country lanes then onto dual carraigeway, 2 mins into that at 70mph, come to a roundabout and change down and the car would go into limp mode, exactly as you had. This occured more and more until every day it was doing it. No amount of stamping on the loud pedal would sort it, but a stop, switch off and back on meant I could get back to normal (obviously though something was not right).
It became a problem on a few occassions when it happened as I tried to overtake, so something had to get sorted out. The dealer scanned the car and nothing showed up, no warning lights and nothing in the error logs, but it had all the signs of entering limp mode, so I assumed that it would be normally something along the lines of a blocked DPF, a sticky EGR valve, MAF sensor issue or leaky turbo inlet etc.
I did all the usual cleaning of the inlet manifold, ran BG244 a few times (which helped), cleaned the MAF and the EGR valve etc but it still carried on doing it.
So, after much reading, I decided to have the DPF knocked out, the EGR disabled and it's been perfect (and a whole lot quicker and more efficient
) than ever..... glad I did it ..
So, it sounds like you either have a blocked (or near blocked) DPF, or the EGR valve is getting gummed up. There are some other areas to look at, MAF sensor (although don't think it would cause your problem), turbo inlet pipe (the big one at the front of the engine) can often be found loose tell tale oil deposits around it will show this, there is an easy fix by using a large jubilee clip to secure it. You could also be looking at blocked injectors, blocked/sticking swirl valves etc, but from what you've described, I'd go first with the DPF, then the EGR and problem areas to look at.
You could run some cleaners through and try the process to do a regen or get a dealer to force one. Or, do the decent thing and get the DPF and EGR sorted out....