My RPG uses Dark Templars as the main character, and after 30 minutes like clockwork. the enemies simply stop attacking, is there anything to be done about this? It's not a matter of losing vision, because the enemy would flee upon being struck, every once in a while they will hit back which would happen if we were allied and I was attacking my friendly computer, but there's not a single trigger that would ally me and this computer? Suggestions, comments?

Possibilities:

1) A trigger constantly assigning alliance status.
2) The computer met losing conditions (i.e., the Defeat action ran for the computer player).
3) An invincibility trigger is on the fritz.
4) A trigger constantly giving the computer's units orders.
5) A trigger constantly moving the computer's units.
6) The computer's units are given to an extended player (e.g., Player 9) and never given back.
7) An AI Script is causing this weird behavior (are you running any scripts repeatedly, perhaps?).
8) A trigger constantly changing the owner of the computer's units.

Run through those ideas and see if there are any issues that could apply to your map, and then verifying that it isn't happening. It sounds like this is a pain to debug because it happens at 30 minutes in, but hopefully you can check for most of these issues in the same run.

Run through Roy's suggestions first, but if you're confident it's none of them, then it's probably this: http://www.staredit.net/topic/5902/

I once had this issue and it turns out it was an AI Script firing too often, after a while all scripts would stop working (couldn't even pick up units in dropships anymore).

Sounds a bit like you've removed vision of himself from that computer

Yes Nude, which is a symptom I observed with my lurkers in that thread. In my case it only affected the lurkers, not other units (as far as I could see), but with a weird bug like that who really knows in what sort of permutations it could crop up under.