I think it's fixed in the new 1.3, it was a problem related to money

But I don't want to disable surrenders, given the AI's tendancy to spam ships&starbases everywhere it was tedious to run them down.

I turned surrender off and don't expect to ever turn it back on. 

Did you get a reply on this? You don't even seem to have very much power at all for it to reset to zero which results in some pretty weird war declarations from puny states that are much weaker than you...

They have forwarded it to the developpers for review.  Maybe it's normal, maybe it's not.  Maybe I don't set the difficulty level high enough (Gifted, in this case), but I would hope that an AI with lots of planets and ships could find a way to adapt to a war.  Ok, in my test save, the Terran AI was facing both Altarians and Yor.  I pushed it to declare war on the Altarians, hoping to catch a break myself.  If it felt it couldn't win, it shouldn't have agreed.  If it felt it was losing to one nation or another, it should seek to re-adapt, either by bribing other AIs to declare war and/or trading for the techs it needs.
In my test save, the Altarians were better than the Terrans because they used beams and the Terrans had Armor defense.  The Yor used missiles. 

Why on Earth would it prioritize Armor when no one near is using Mass drivers?

Why is it unable to adapt to changing conditions?  Again, is it because my difficulty level was too low or is because the AI's inability to correctly use the planet's bonuses impeed its development from mid game on?

I have no real answer.  Given other bugs, I'm disinclined to play another game to test it right now, I'd prefer to wait for the next patch.

I am beginning to wonder if sometimes the AI isn't sort of, falling asleep, or it becomes totally broke and is unable to do anything.  Or coercion penalty is preventing the AI from adjusting?  I remember a game where the AI wasn't doing anything anymore. No attacks, no invasion unless in direct line of sight.  At some point, I gave the AIs some money and some techs, and suddenly, for a few turns, the AIs were very agressive and even conquering other planets, then they fell back to default behavior, not doing squat.

In another game, I was very close to the Krynn when I started and they kept attacking and invading my planets pretty soon after 1st encounter and rapidly developping.  It's like sometimes, it's totally blind and then cannot adjust at all.

I don't really know what's going on.

Yeah. It seems like there's a moment in most of the games I've had surrenders switch on, where it's like there's like the 'Age of Surrender' and within the space of 30-40 turns, the number of galactic powers goes from 4 or 5 civs, to 2. This would be fine if those surrendering were like 1945 Germany, but I've had some really ridiculous things happen. In one game, I as the Altarians and the Iconians were at war with the Drengin. The drengin were quite a ways to the north east, and the Iconians on the directly opposite side of my territory (behind me, for the Drengin). Several turns pass, and I'm actually making progress into Drengin territory having even taken a few planets. Having an exploration treaty with the Iconians, I note that they have a poor military, but there are (from what's visible) no Drengin ships, and certainly not any Troop Transports. A few more turns pass, and I take yet another Dregin planet. The following turn, the Iconians and two other races surrender to the Drengin. I quit and started a new game. Not rage quit, more like 'wtf? quit'. I think surrenders need to be examined and made more logical.

That is possible, I have seen the ai be at war with two people but have all its fleets on my border while we are at peace.

Well I do have snuggler Colonies. I guess I'm very cuddly. ^.^