Oh yes, they can be soooo nice when they want to be it's all part of the cycle to keep you hooked in.
It's often said on here that if guys like this were total arses 100% of the time no-one would stay with them for five minutes let alone years.
It's the 'jackpot pay out' fallacy which I've fallen prey to myself more than once.
He's been charming and nice and supportive before so when he's utterly horrible to you there's always that part of you that thinks ' oh, this isn't really him, if I just do xyz/twist myself into a pretzel shape the old, loving, great guy will come back'
Often though, the Mr Nice Guy never existed in the first place and 'nice, loving etc' was just a front to hook you in.
Gosh, I probably sound a bit bitter and twisted here but i'm really not, just acquainted to some of the tricks through personal experience.
I remember so clearly the guy I was with for seven years who would turn on a tuppence and be jealous, terrifyingly angry, threatening, shouting, screaming, controlling and then the next minute be Mr supportive, kind, funny, caring...
Still remember justifying this relationship to friends who could clearly see he was an abusive arse with statements such as 'but he's lovely 80% of the time, it's the 20% he's horrible to me I can't cope with.
With time and hindsight and spending time on MN i've come to see that 20% of horribleness towards me is just way too much and nothing less than 0% is acceptable.
Well, I have hungry cats to feed but here is the link to that thread (dealing with abusive relationships in general not just emotional, must correct myself there)
Have a look, have a post if you feel like it, everyone there is utterly lovely and many are battling with similar situations to you right now.
www.mumsnet.com/Talk/relationships/1333564-Support-for-those-in-abusive-relationships-thread-6