I've had a few theories when it comes to explaining this phenomenon. I have deja vu a lot. It flares up and goes away. As often as a few times a day to every few weeks. I have read that simple partial seizures are accompanied by deja vu. Other symptoms include hallucinations (the prior experience in this case), muscle spasms/twitches and a few other things. All three of the named symptoms apply to myself. Also it has been documented that a strong sense of deja vu occurs immediately before an epileptic seizure. I had the feeling that the part of the brain that processes memory - the hippocampus - could partly be to blame. It may misfire and confuse two similar scenarios and make the person experiencing believe that they were the same.

One thing I have noticed personally, while reading various comments, is that a few people who experience deja vu often have bad memory, so it could be a problem with the hippocampus. Confirmations? Rejections?