I hear ya expat. . .I guess I've been lucky that the OOM problems were never an issue with FSX. . .for me at least. Other odd behavior and what I suspect were problem gauges that over time just became unusable. The fresh install, for the time being anyway, has calmed things down significantly.
In the case of P3D_V4+, I haven't started it up since before Christmas, about the time I went to XP-11. I was really impressed with the Sim and the realism and detail. My enthusiasm for it waned considerably though when I found that whatever you knew about FSX and P3D as far as setting things up to your liking. . .is nothing like XP-11. . .also, for me, I was excited to be able to move my sceneries over to the new sim. . .nope. Scenery is handle completely differently and nothing that was done for FSX is even remotely compatible (that includes aircraft). Their method for building scenery is laborious, and that's only my opinion because I am so deeply seeded in the way I've done it for years and I'm not interested in learning another system at this point. So I haven't been back to XP since April I think.