ohlins rear shock problem

[reeve]

hi, on my 06 ec200 i noticed the front end acting crazy when blasting over rough ground, so i adjusted all the suspension settings to standard. most of them were already standard except the rebound on the rear shock which was all the way out, so the shock rebounded really fast. after sorting this the bike was perfect for a couple of rides but then the crazy fork actions came back. i have just checked the shock rebound settings again and it has backed itself off about 15 clicks. Is this a common problem and what can i do to stop it??? thanks for the help.