My flatmate is Dr. Jekell and Mr. Hyde. We were friends for a year before I moved in...?

Once I moved in, things have been fine, but about once a month he gets in a funny mood and storms around in a strop. I've always been consistently nice to him (offer to cook a couple of times a month, always share my booze) and in return, he does all of the hoovering, basic shopping maintenance, etc, but half the time I don't know what sort of mood I'm going to be greeted with when I walk in the door. When we're ok, we're very ok. When we're not, I want to drive a stiletto through his neck. I've been there for 6 months and although I really don't want to move out, his mood-swings make him hard to live with. I don't want to lose my friend, but it seems like I can't do anything right in his eyes half time. When he does something stupid or offensive, I always let it slide. When I make a mistake, he tortures me with reminders and sarcastic little digs for weeks on end....HELP! Should I just drug him and be done with it?