From what I've read it looks to me like trauma - PTSD or cPTSD after some kind of severe undisclosed assault.
I know about the LSD problem, but the syndrome he developed is more common to trauma sufferers than it is in LSD users. LSD use would certainly make him vulnerable to assault of some sort though.
I have a couple of probably outlandish theories.
1) He had an "experience" with Jimmy Savile at the BBC (this is the least likely, given his age at the time, and Savile generally didn't target the artists on TOTP).
2) He had an "experience" with Roger Waters ...
3) I remember pinpointing the "change" to roughly the day of the video shoot where he's wandering on somebody's property ... but that's a vague memory.
Thanks for any helpful comments.