Hello, I am new to this forum and kindly ask for your opinion and/or advice on the following issue: I am running a Mac Pro, 2 x 2.8 GHz quad core Intel Xeon (early 2008), (MacPro3,1) and recently changed RAM. I purchased the prescribed type of RAM boards ....DDR2 FB-DIMM 800 MHZ. 8 x 2 Gb Boards - so the thing should sport 16 Gb in total, using all available slots. However, it is not so: Riser B usually shows slots 3 & 4 as being empty. On some days slot 4 only is indicated as empty, slots 1, 2, 3 are indicated properly as occupied. So, usually I am seeing 12 Gb of RAM, on a good day 14 Gb, but never 16 Gb as it should be. At any time the clocking is down to 667 MHz instead of 800 MHz. I experimented a lot - changed the RAM boards around, put them in different combinations and so on. You know the ropes All RAM boards are in good order, but I am still getting 12 GB to work with. Two observations strike me in particular: 1.)When removing riser B entirely, just using riser A with any combination of four RAM boards, it shows the right specs .clock-rate is correct, and so on all is fine. 2.) Using both risers and putting two of my old, original (Mac) 1 GB boards in slots 3 & 4 on riser B in addition to two of the new 2 Gb boards in slots 1 & 2, then the machine shows the correct amount amount of given RAM, but the clock-rate is again wrong. Although all of the boards are definitely 800 MHz boards. What gives?? Also, there is this red light on riser B occasionally, sometimes not. Could the riser board itself be faulty? If anyone has any experience with this annoying behaviour - I would really appreciate your input. Thanks.