Keep a blood pressure tester with you and when you feel dizzy slap it on and take your pressure and pulse. First lets find out if it is going up or down when this happens. Checking your temperature is also a good idea. For a few days, check your pressure when you get up in the morning, and randomly during the day, and when you are resting before going to sleep so you can see what your normal pressure and pulse usually is to start with.
Lots of things can cause dizziness, and they vary from person to person. When the doctors can't find a physical cause, it is usually from environmental reasons, or your diet, causing a hormonal imbalance, or inner ear problem that is difficult to detect, or other things I haven't listed. You are not the only person with this problem. Kidd57 has also posted a question because he has the same problem, and very good health insurance.