I have seen posts something to this effect before, so I had kind of kept an eye out for something that might be triggering it. Weeks ago, during a fight with the spider spawn in Sielwood I happened to notice that I was getting pummeled a little bit more than usual. I opened my char screen to see that my ac that was supposed to be at 23 was at only 20, and then when the Helm of Armor wore off I was all of a sudden at 17. This was just the first time I had seen it in action, I wasn't really sure how it happened but I think I figured it out today.
I will go step by step through the fight to help you guys figure out what is going on first of all my AC at the beginning of the battle was 26 I have a Helm of Armor II now and used it just before I engaged the spider spawn. This time there were two Aranea and a Giant spider, I had my char screen open so I could watch things more closely. The battle was going fine, I hadn't attracted the ettercap into the fight this time so after dispatching the other three I headed over to him. As he saw me he cast web, and being the fighter that I am I of course failed. So I got caught in the web and my ac drops to 25 because of it, which is normal, and the ettercap decided to try his ranged touch attack, and that I ALSO failed.
Here is where the problems begin, you see after that second failure I dropped another 6 AC somewhere. I know around the same time as the web, my mage armor wore off, and I lost about 4 or 5 AC right there. And then it seemed to take another 3 AC off when the ranged attack wore off, and then another three when the barkskin wore off. I don't know if the math works out, but that is how it seemed to happen. Right now I am sitting in town naked *shivers* and I have a natural AC of 6 when I should have 12. 10 + 2 for the 15 dex I have.
ok let me see I had 26 ac, lost 1 from the web, and then four or five so that puts me at 20/21, then lost six so that puts me at 14/15...hmm yeah that does kind of work cause I have 15 ac fully clothed right now, and I think I should have 20. Oh man just noticed my reflex is usually 4 but it is at -2.
Hmm ok so lets see...now....if I relog.
Ok relogged my natural all clothed AC is 20, which is normal, and my unclothed is 12 as it should be, and now showing my reflex save at 4.
I hope this helps somehow, and just so you know, through all this my character stats, str dex etc all stayed the same I never got hit by poison or anything. The only thing that changed, and I noticed that in town was my reflex save....well and the whole AC thing of course