ok, I'll try and explain episode 5, but im not sure exactly whats going on now
In the copter en route to the freighter, they go slightly off the bearings given and Desmond begins to time jump.
He is in 1996, back in the military, and thinks he had a dream of being on a copter. He jumps back to Desmond in 2004 (present) and doesnt know who anyone is or where he is. It seems his awareness, his consciousness, is that of Desmond from 1996. He continues to jump back and forth, and while in 2004 he speaks to Daniel Faraday who tells him to go find him in 1996 at oxford uni where he is working on a time travelling device that "unsticks" people in time. So its not a persons body that travels but their consciousness. He finds him and is told that in order to survive the ordeal and not have a brain aneurysm, he must find a "constant"...something or someone that is familiar in both times. This is for Desmond, Penny. He tries to contact her in both times, eventually gets he rnumber, calls her from the boat as 2004 Desmond and all is well
Then we see Daniel back on the island, flicking through his journal, and he finds and old entry - "if anything goes wrong, Desmond is MY constant"
Now my theory is that in order to prevent a paradox, the universe has "course corrected" and disturbed Daniels memory in order to stop him remembering meeting Desmond back in 1996!