Thought I would share a similar experience with a co-worker, SJ… although I wouldn't deign modify the venerable Sqad's diagnosis.
The friend had experienced similar problems for quite some time but was especially subject to vertigo with any sudden head movement. He discussed the issue when visiting his chiropractor. The chiropractor preformed a simple test involving head positions and immediately suspected BPPV. The specialist then did a manipulation of friend's head best described as suddenly rotating his head (I guess you had to be there) but the relief was immediate. That was 6 months ago or so, and while he returned to the chiropractor for a check up no other treatment has been required. He is joyous, of course. Might be worth looking into.
More here about the crystals in the middle ear as the culprit:
http://www.michiganear.com/bppv-fact-sheet.html