i wish you were in Nottingham, i'd hook you up and have a look for you.
seeing as it is random and intermittent, and the limp mode does not remain an ecu condition once the fault is detected, i dont believe it will be a serious issue or major component failure, otherwise i would expect the limp mode to remain until being tool reset with either fiatecuscan, alfadiag, or examiner.
i suspect its more likely a poor connection, maybe a sensor breaking down at worst.
i'd go back to basics and start looking at all the battery connections, main earth connections, main engine bay fuses, relays, and sensor connections such as thermostat, fans, oxygen, etc etc
or, once you've checked the obvious and the large, if you've still notmfound anything
invest in a obdII cable, from either ebay, or rarebear on here does modified cables that will also connect to the abs and airbag ecus too (it is easily modified with a steady hand and a soldering iron) download the free version of fiatecuscan or alfadiag, and have a play yourself.
once you have some codes, you'll be able to reset yourself, but more importantly post them up in a fresh thread title, and i guarantee some boffin will translate it to real world speak for you! and point towards the right component straight away.
you'll be surprised at how much you can achieve yourself, i'm now plugged in to fiatecuscan weekly for a fix of "no fault codes", its very satisfying!!!!
sorry i don't have an instant fix idea, good luck an happy hunting!