As mentioned above, a possible cause is stress/anxiety. I had a really bad time of it a few months back with almost constant dizziness and the doctor now thinks it is caused by anxiety combined with a jaw clenching tendency (brought on by stress). I have since identified a lot of the causes and removed them from my life and am making a slow but steady recovery.
Try to pay attention around the time you get dizzy as to whether you have been taking fairly short shallow breaths and also whether your teeth are together (sounds silly but it really does affect some people!), if they are then drop your jaw and relax.
Also practice deep breathing and go get a nice relaxing massage
Jimmydacraw