so... I tried a few things. Since the last big mod I installed was Rend's Germany terrain, I tested on my SF2NA mod folder (which I didn't use for a while)
I flew the stock Germany terrain, enough times and various missions to see if it would crash or if my aircraft would go all "transformers" on me.
Nothing...
I install the Rend terrain, fly it, and it freezes. Sometimes in the cockpit, sometimes when there's a lot ground objects being attacked.
I rename the Rend terrain (so that it's not recognized), install the older four seasons mod for GermanyCE, fly a couple of times... and after a few missions comes the dreaded freeze. (my mind is boiling now)
Out of curiosity, I delete the Rend folder completely and leave the four seasons mod. Fly that, like at least ten missions, no freeze, no transformers, no blank terrains.
Weird (or what?)
Since I mainly use the SF2 mod folder for almost everything, I delete the Rend folder there, reinstall the four season mod and likewise, no freezes...
I tried my Desert terrain (modded), seems ok. I did find the TERALPHAOBJECT.FX and TERSOLIDOBJECT.FX in there, which I deleted. Maybe those were the culprits that made my missions freeze in the desert.
Still need to test the Vietnam and Iceland terrains. And fly a full campaign.
If nothing weird happens, I'll try updating my driver and see what happens.