Problems with G5

I get unexpected problems with a dual 1.8 G5 (2 GB RAM, 2 disks inside, both 320 GB). In fact, the machine seems to break any hard drive I plug in it (via USB, eSata or internal HD). The internal hard drive hosting the system is clicking every five minutes or so, and two external hard drives are now mostly unsable (they unexpectedly unmount when copying, or they just don't mount at all). I checked them all with DiskWarrior. Some times, they can be repaired, sometimes not.

The problems appear with Tiger and Leopard as well. I lost Time Machine backups form an external hard drive (Seagate 320GB, got "keys out of order" error message in DiskUtility). The other external HD I have to put my photos (750 GB Seagate) can be used in both eSATA via Lacie PCI card and USB. Both USB and eSATA fail unexpectedly (unmount while copying) and everytime the hard drives seems to fail more and more everytime I try to use them.

I used Apple products since years '80 (SE30) and did not have problems whatsoever with previous machine and my actual macbook Pro. This G5 is getting me crazy, and I don't want to put hundreds of dollars repairing since it is worth less than 800$ actually... Is it the power supply, the logic board, a problem with electric voltage up and down? I always found solutions to problems but now, it is the first time I don't see the light... I feel like everytime I plug a hard drive in this G5, I play with fire and may loose my backups and archives!

What do you suggest? Anyone got these problems with a G5 machine?

Thank you very much for any support!