I get the same thing. Instead of trashing a plist and a cache, I tried simply resetting my preferences. They seem to stick for awhile and then revert. I can't explain it.
All I do is reset my prefs; it's easier than and just as effective as trashing stuff.
I think that I have found the cause. I checked my Flash preferences and they were OK (Block all sites from storing information on this computer). I then used Flush, which found no Flash cookies, of course, but deleted the relevant preference file. Immediately, my Flash preferences reverted to default (Allow sites to save information on this computer). I reset my preferences and then used Cocktail (via its Pilot setting) to clear Flash Player caches and Flash Player cookies. Again, Flash preferences reverted to default. I reset them and they seem to be holding.
