T3400 stopped cooperating with its video card

Hello community,

I am stuck with a rather weird problem. I am using a Dell T3400 with a GeForce 9600GT video card. It worked without problems for some years so far. However, yesterday I needed a machine to test another video card and used the T3400 for it. It also involved powering the T3400 with another PSU (BeQuiet E10 400W) and plugging in the 9600GT into another machine.

After the testing the T3400 refuses to cooperate with the 9600GT. Following observations:

1. When booting with the 9600GT installed the T3400 sometimes gives the LED error code 1-3-4 (which means "A possible system board resource and/or hardware failure has occurred."). It then tries to reinitialize several times, during the 3rd time the CPU fan gets very loud and stays this way till I power off the machine by force. Sometimes it seemingly boots up properly but the video card sends no signal to the monitor so it stays in standby and shows no output.

2. When inserting a different video card (an old X1800 XT I have lying around), the T3400 boots up just fine.

3. When inserting the 9600GT into another machine, it boots up just fine.

4. I tried inserting the 9600GT into T3400's second PCIe slot. It booted up fine. Then I shut it down and left but when I tried it again half a day later it again showed the same symptoms as the first PCIe slot.

5. CMOS reset by removing the motherboard battery didn't help.

Now I'm somewhat stuck since the error code is rather generic and I'm completely blank regarding what might be the source of the problem.

Any input is appreciated!

PhotonX