dizzy lizzy

I have had the same problem for the last 6 years (I'm only 20), but nobody seems to be able to diagnose me. It comes and goes with varying degrees of severity, but I never know when it going to happen or how I can prevent/ help it.
Here's the problem: Sometimes, for no apparent reason, I get really dizzy and unstable - I feel really faint and more often than not 'fall' into the nearest wall or tall object to try to balance myself (or just to lean against until I feel better). There also seems to be a general weakness, both of body and spirit. My mood also changes - if I feel REALLY dizzy, I get really quiet and withdrawn, barely responding to the people around me, but sometimes I can be pretty stubborn if anyone tries to help me (I ignore their offers of help and attempt to walk off in a straight line... except that doesn't normally work out very well!) Duration of my problem can be anywhere from a few seconds to a good twelve hours, but the average is maybe an hour or so. I'm not dizzy exactly for an hour, maybe just 30 mins, but I still have the other (mood) symptoms after that.
I've noticed that there are several triggers; bright lights (including the sun), flashing lights (strobes are the worst, but they don't even have to be flashing that fast. The worst time I encountered these lights a colleague was able to tell me that my eyes were rolling in different directions, and she didn't need to ask if I was feeling better yet 'cos she could see I wasn't!), and physical exertion. Even if I drink loads of water and have eaten properly, nine times out of ten I will have problems after playing football, running or doing karate or whatever, even if it's just been for a few minutes. I'm always fine as long as I keep going, it's when I stop that the problems start!!
However, I have also experienced problems without any of these triggers - just by walking across a room or sitting at a desk.
The whole thing is getting out of hand. It can be so dehabilitating at it's worst, and just plain annoying at it's best. I REALLY don't want to stop doing sports, and having to avoid flashing lights is a pain, especially when where I live the police cars all have white strobes rather than red-and-blues, and the lights where I work are never switched on and flicker for an insane amount of time before they settle down. It also stops me from going out places I'd like to go (that I know have flashing lights), just because I don't want to risk having to depend on my friends/ colleagues to scrape me up off the floor and get me home...! It's not fair on them.
Please, if anyone has any ideas on what might be causing this, or any suggestions that may help me deal with it, they would be greatly appreciated. I have been to a couple of doctors over the years, and they don't seem to know. The most anyone commited to saying was first Meniere's (which was then ruled out by another physician), then epilepsy (which was never followed through). Its really hard having to explain to people whats wrong with me when there's no diagnosis! I've had loads of tests including an MRI which have all been normal - blood sugar, blood pressure etc were normal during episodes too.
So I was wondering if anyone else has had a problem like this, or whether you think it [I]is[I] epilepsy or not.
Thanks!!