Sorry you're having such a rough time. These things do happen, and just to put it in perspective our 500 TA (85bhp) was problem-free for over 3 years / 35,000 miles, as many others have been. However it's more about what processes are in place if something does go wrong, and this is where Fiat and a large number of its dealers are gravely lacking.
Whilst our 500 TA was absolutely brilliant (as were 3 other Fiats & an Alfa), my Sister had a Alfa MiTo TwinAir (85bhp) which was an absolute pig, with numerous problems including two which were very similar to yours - on one occasion it just cut out on a dual carriageway at 70mph and she had to try & coast it into a layby up the road. It'd previously had a new UniAir control module after the first failed at 10,000 miles.
We went back & forth so many times to the dealer that I've lost count, and even when we tried to get shot of the thing back to Alfa, they dragged their heels - the dealer didn't seem to know what they were doing, the finance company (FCA) were OK but not exceptional & Alfa UK were absolutely appalling. The only saving grace was when Italy got involved - they were the only ones who seemed bothered about the customer.
Thankfully, since October last year, new consumer laws have been introduced which replaced the previous somewhat vague stuff with what looks like pretty strict timescales/opportunities for a retailer to rectify a fault before the customer can demand a refund/replacement. A summary of the new guidelines is here
http://www.bbc.co.uk/news/business-34410782 but having read it, it looks like you're well within your rights to demand a replacement car now - previously there was a slight vagueness surrounding how many chances a retailer could have to try & resolve a problem, but this is clearer now.
I think you need to write or email the finance company, saying they have 14 days in which to inform you when the replacement car will be forthcoming. By all means phone Fiat UK & open a case, but in my experience very little happens in between phone calls & you'll end up getting extremely frustrated by them not knowing their ar*e from their elbows, so FCA finance are probably the way to go for the official stuff.