@Seekermeister: To answer all of your questions:
1) The contacts are looking fine to me, no signs of cracks or other kind of "interruptions"
2) Nope, the fingers are completely hidden inside the PCI slot of the motherboard.
3) The motherboard seems to be very well installed and it's firm to the case. I've never moved and/or replaced it thou, plus the computer is kinda new (Jan 2010).
I have an update anyway.. I was putting a little pressure just to see if it was a matter of contacts (still, the contacts are completely hidden and the X-Fi looks perfectly seated in its slot) BUT i did try something else... I've tilted the sound card a little bit, putting a little pressure towards the bottom of the case instead of pressing towards the mobo and.. It worked again. So (i know this is gonna sound super crazy) i used a platic tube to help it stay down. One end of it touches the corner of the processor fan, the other lays on a little screw on the card itself.
I DO NOT want to use it in this way, that was an experiment only. I was wondering: Maybe cleaning the sound card contacts? Could it change something? I'd need to know a brand for a electronic contacts cleaner so i can buy it and try.. Then, if it's not gonna work with the card freshly cleaned simply seated in its slot without plastic tubes or straws that would be great and all of this big headache will end with the simplest solution ever.. If not, i'll work my way around to return it and try with a different one...
Thank you, really appreciated.