Every now and then my char starts to move really slow, like when you suffer from freeze shock (or some name like that) and takes 90-100 dmg/sec. The amount decreases as my health decreases and it stops a few seconds before killing me. I have absolutely no idea what is causing this, it started while I was in the middle of a dungeon (the last one, floor 32) I've already been through many times. What the hell ???

The most likely explanation is that you're on fire, or poisoned.

There *might* be an enemy trapped in the floor or something below you that you cant see that is attacking you. Had it happen in TL once, and other RPGs a few times..