This is probably not very helpful, but just to relate an odd experience I had:
Running a few WordPress sites on my VPS..had some high cache levels for several months. I tried several restarts, adjusting PHP Versions, other random things here and there...
A few weeks ago (I think it was a few weeks ago,,cannot remember the exact date) Dreamhost was having some server issues. My sites were all down for several hours. When they came back up, lo and behold my cache level was way down (riding under 250mb) and since then has been staying that low.
I have no idea what occured to reset my RAM levels like that, but I hope it sticks!
My sites are all in Irvine, CA
My thought is that if you wanted to reset, you could request for your vps-server and database-server to be moved to a different data-center, which might possibly result in a kind of reset...
This is pure speculation, but if you have tried everything else then you probably don't have anything to lose by it.