How to write a volatile (or mercurial/changeable) character (characterisation)?

I've been doing more writing lately than I had been previously (due to actually having acquired some inspiration from somewhere, not sure where - but hey, we don't question these things ^^*). I'm having a bit of trouble writing a character who I intended to have quite a volatile personality- very changeable, happy one moment and then angry at things which are really just insignificant in the bigger picture- but they're coming off as being a drama queen/ poorly written character. Does anyone have any hints on how to stop my poor character seeming like he's just OTT bad? Sorry my question isn't too specific, this guy is in the planning stages still... Plus, before anyone asks, no, he is not the brooding, anrgy type. Just the type that is either very happy or very angry most of the time. :) Ta people!