The problem seems to be gone with 0.95.4 #12462 x64. Or at least, it's much better now. Thank you.
I've been watching the process for almost an hour and the private bytes size stayed at a relative constant level.
Since in the new build, the ConnectionNotify plugin seems to be broken and to ignore all filter rules, I get masses of popups for connection notifications and (I think) therefore the used memory is going up and down all the time, but as I wrote, it stays on a constant level.