Is he lying to me or playing around or really sick?

It doesnt sound like bipolar, not if it only comes around every couple months. I think for the most part he truly isnt happy. He could be doing things on the side, and when they dont work out is when he decides to stick around until the next time. Hes probably feeling bored, and wants to venture out, and explore new territory.

The only thing you can do is have a serious talk with him, because only he can give you the answers your looking for. Dont let him tell you everything you want to hear, that isnt going to solve the problem. Tell him you want the truth, because you deserve it.

I think what you should do is that you should sit down and talk to him and tell him how you feel? About how he does these mood swings to you. The best thing to do is give him some space, tell him how you feel and how you dont like what he does, and then just give him time to think about what he does to you. Or comfort him more, making him less stress about his problems and sutff, and if that doesn work and he keeps doing this to you then, honestly I dont think you should be treated like that, and I know that you love him but if the love of your life is going to keep doing this to you and keep playing games with you then you just need time alone.. Hope I can help. :)