Bad Memory Riser?

Hello,

I recently installed 2 new 2GB modules into my Mac Pro for a total of 6GB of RAM. I followed the Apple RAM installation guide, and everything worked fine for about a week. However, I booted up this morning and noticed it recognized only the stock 2GB instead of 6GB. I shutdown my Mac, reshuffled the modules, and it still only recognized the original 2GB. I then placed all four of the dimms onto one riser (I'll call it riser 1) and left the other riser (2) empty. Upon restart, my Mac recognized all 6GB of RAM. I shutdown again and physically swapped the two risers whilst keeping all RAM on riser 1; that is, riser 1 moved into the lower riser bay and riser 2 moved up to the top bay. Again, this worked and my Mac Pro recognized all 6GB.

My questions are:
1. Through a process of elimination, this seems to be a problem with riser 2, correct? It isn't my motherboard? The other RAM--OWC, lifetime warranty--tests fine in either riser of my friend's Mac Pro.
2. Is it harmful to the computer/bad to keep all four dimms on a single riser and leave the empty riser in the machine? I understand it isn't best for performance, but is it harmful?

Thank you all in advance--this problem has me quite upset!