I have this problem when the road gets really steep.

But, I read somewhere that if the bike isn't fit properly that this will happen. I believe it was something about if the bike is too long that your center of gravity is too far back, causing the front end to pick up on hills. It also affects the stability going down hills. So, besides what Knotted said, you might want to check the fit (or wait until one of the gurus comes on and tells me I shouldn't believe everything I read? I can be so gullible )