This only happens to other people

Even though we often report problems and breakdowns of machines, until now we have been little affected by such problems.
This period of normality ended this weekend. We relate the facts to you and the way in which they appeared with an aim of recalling the origin of this problem in detail - such a procedure leading us almost to a diagnosis.
The initial problem that we had on this machine was an increasingly frequent tendency to refuse to wake up and instead to reboot. We ignored this problem and it was the only one which we had to complain about with this Mac Pro.
However this Saturday, we have decided to start intensive tests with the idea to publish them. During one of these tests, particularly heavy on the RAM, the machine had a Kernel Panic that the logs associated with a memory card.
We then started by installing a clean system, however when the machine completed this some hours later the situation was the same.
Thus we thought that the RAM that we have added created problems and took it out. To test : we started doing the hardware tests of Apple available on the installation CD of MacOSX by hold the key D
on booting the machine.
In spite of the fact that we returned to the original configuration (4x 1GB Apple memory), and that we put all the original equipment on the machine (engraver, hard disks, video card...), the situation did not improve, the machine abruptly restarting during the tests.
We nevertheless continued to test all the possible pairs of memory cards and swapping the memory riser cards, the situation still degraded.
At the last restart, the machine made a kernel panic when it reached the desktop, and since refuses to start up at all.
On pressing the start button, the fans start up, but nothing else. One sees just two red LED on the motherboard between the two memory cards. We did not succeed in finding their significance but the red is seldom the color of the good news.
We presume that the problem comes from the motherboard (certainly) or perhaps from one of the two memory risers.
If we had to bet, given the rapid progression of this fault, we would guess that it is related to the one of the connectors of the mother board on which the memory riser boards are fitted. Our actions of continually plugging and unplugging the memory cards have probably broken the bad solder joint.
The machine will be deposited at the after-sales service this morning. We will keep you informed of progress, Attention, this does not put in doubt our thought on the MacPro. It is by far the best Apple machine and probably the best computer in this category. They are well conceived and the finish is of good quality