Ati card working at extreme temperatures, who to blame?

For those Who dont want to bore themselves reading all this, a brief resume of my problem: I have an Ati X1900XT, started giving me artifacts, I checked temperatures they where way high (80C!176F!), I used atitool and gave me artifacts, I upped fan speed, could play COD4 with no artifacts, and Ati tool test ran stable for 15 minutes, and now temperatures are higher and Im scared that permanent damage could be done to card, problem showed up when playing assasins creed, already check fan and no dust, Im worried about the problem and would like any advice on what I should do (After market heatsink, replace card?)

Hi, I own a system that has a Sapphire Ati X1900XT which I bought a year and a half ago. I almost never turn the system off, not even at night, so the system is always on and working. But recently I started having problems with the card, first while playing Call of Duty 4, I started getting some artifacts and I noted the card's temperature was really high, dont think I havent googled this problem, and the temperatures I was getting were way high! (almost up to 90C, 194F while doing Atitool artifact scan, and 80C-85C, 176F-185F when in COD)

I first decided to open up the system and check if the card had any dust or any obstacle that was preventing the fan from doing its work, and I found out the fan was very clean, although I did not check if the heatsink was broke or loose, Im doing that today although I dont think that is the problem (how could the heatsink get loose, if it was working perfectly before)

I also downloaded atitool, to check artifact scan, and started getting tons of artifacts after a while in the test, when the card started to heat up. Therefore I decided to check for fan speeds to see if I could manage something in those settings to help up the dissipation of heat from the card, so I decided to choose a fan speed of 100% if the cards temperature surpassed 80C 176F, at last I could play COD 4 without problems, and the atitool artifact scan lasted for at least 15 minutes, with a stable temperature all the time, although I cant deny it was really high for this card (83C 181F throughout the whole test)

So really my point in coming here and asking for your help, is to find out what the culprit of such high temperatures could be: I think it may be the fan that is failing due to the high use throughout the whole year, or may it be the thermal paste in the heatsink?

Also what are the possibilities of permanent damage to the card?, yesterday when I started playing Assasins creed the PC restarted itself 2 times, and I noticed once I entered atitool test it had tons of artifacts, although today I turned on the computer and ran ati test for some minutes and no artifacts appeared, although I stopped it because temperatures where getting to the boiling point of water! also I have to note that the fan started sounding like it was loose, Im going to check that today also. Any help its appreciated.

What course of action should I take?

Should I buy an aftermarket heatsink? or send the card to replace for a new one?

Edit: I forgot to say the card is not overclocked and idling at 65C 149F