MSI P35 Platinum + Q9550 CPU + 4x1GB RAM

Question

Greetings!
I just upgraded my system by changing just the CPU, from E6550 to Q9550, and having some small problems.
The past:
My system was stable with the E6550 and the 4x1GB RAM sticks (Kingston, 667MHz), with the only negligible issue being that during POST it momentarily stuck (e.g. for a fraction of a second) at ~3100MB before sweeping the entire 4096MB of memory and proceeding with loading the OS (Win7x64). I've never had any weird BSOD issues and never done any OC (on CPU, RAM nor VGA).
The present:
Problem is that, now, with the Q9550, system wont POST with all four of the 1GB sticks plugged; it stops/freezes at ~3316MB (if I hit ESC or DEL during RAM-sweep it blacks out and goes off-power)  before the "beep". Needless to say that I can't even access the BIOS menu in this configuration. Removing one 1GB stick, specifically from slot #2 (counting from the CPU), thus running at 3x1GB, allows for normal boot. In this configuration, the new CPU model is properly recognized by both POST and BIOS, but the CPU multiplier is reported wrongly in the BIOS settings (x7 instead of x8.5), just like it "remembers" my old CPU's setting. How can I check, within the OS, that the frequency is the right one? Many programs report it correctly (e.g. CPU-ID or Windows), but there's the chance that they're just stating the nominal spec of the CPU model...
Any clues on how to address these two problems?
My (small) troubleshooting:
I am wearing the latest official BIOS, version 1.C (or 1.12, dated 2009-12-30), that hasn't given me any cause to worry about, in the last 3 years.
The system worked fine before (w the E6550 CPU) on the 4x1GB RAM, so I am kinda excluding the case of a "bad" slot or stick. I haven't (and don't wanna) undertake the tedious task of trying all possible permutations of 4 slots & 4 sticks. Please note that the RAM sticks are same brand, size and frequency, but the model-numbers might not be exactly the same (one pair was bought in 2007, the other in 2011).
When booting with 3x1GB RAM, and running some benchmarks from the OS (PassMark PerformanceTest, Prime95) reveals that CPU is working fine. Only problem is that its temps are a little on the hot-side (40-45C@idle, 65C+@load, at ~25C+ ambient, on stock cooler) and the CPU benchmark scores are 5-10% below the average-baseline; however, these issues worry me not because the CPU was used (got it from eBay for $90).
My PSU is a 550W HEC, and the rest of my system comprises a HD4850/1GB VGA, two HDDs (WDs, one of them a "green"), one SSD (Intel/60GB) and a DVD-writer.
Any ideas?
I am hoping that there's some BIOS tweak I can make in order to re-recognize all my RAM with the least possible hardware changes and operations. Maybe re-flash the current BIOS, or try to roll-back a version? Will tweaking the RAM settings (voltages, freqs) help any? Is that do-able on the P35 board? My last (money-costing) option is going for new 2x2GB RAM sticks, which I evidently want to avoid...
Thanks for your help!

Answer 1

OK, I've had some time in the weekend and I did my tests, that seem successful Yay!
A. New CPU multiplier mis-identification in BIOS (7x instead of 8.5x) -- This was fixed by clearing the CMOS (as RemusM suggested, using the on-board button) and then "Loading Optimized Defaults" by the main BIOS menu. Apart from the time/date and boot-order, the only other thing I had to change was the On-Chip ATA Devices RAID mode (enable AHCI), to allow the SSD drive where my OS is stored to boot normally.
B. POST stopping with 4x1GB RAM sticks -- The above BIOS-related changes didn't help here. So, I undertook the task of checking each RAM stick alone with MemTest86 (as XFM suggested). To this end, I plugged each stick in Slot#1 (DIMM_A1) and no errors were found, for a one-pass test, for all four sticks; evidently, my RAM sticks were fine. I then tried some permutations of the four sticks and the four slots and, lo and behold, one of them, worked! Please note that my four RAM sticks were purchased in two kits of 2x1GB, that were equal brand/size/frequency and the serial numbers were almost the same (apart from the last 2-3 digits in a series of 10-15 characters). The sticks looked similar even to the bare eye (very similar PBC "routes", chip placement etc). The only difference was the date, 2007-vs-2011.
For what it's worth, the non-working combination was:
   DIMM_A1 : Kit#1 (2007) / Stick#1
   DIMM_A2 : Kit#2 (2011) / Stick#1
   DIMM_B1 : Kit#1 (2007) / Stick#2
   DIMM_B2 : Kit#2 (2011) / Stick#2
while the working combination is:
   DIMM_A1 : Kit#2 (2011) / Stick#1
   DIMM_A2 : Kit#2 (2011) / Stick#2
   DIMM_B1 : Kit#1 (2007) / Stick#1
   DIMM_B2 : Kit#1 (2007) / Stick#2
(i.e. the two sticks of the same kit were placed in the same RAM Channel slots: the Green and the Orange)
Nevertheless, one mystery remains unsolved: In the fast-sweep of the RAM during the POST, the check still stops/pauses at ~3320MB but, fortunately, it proceeds to sweep the entire 4096MB and then boot the OS normally! The duration of this "pause" is maybe a little longer than it used to with the old CPU: it now last up to one full second while it use to pause for only a fraction of a second. I can't help but correlate this pause location (at ~3300MB) with the maximum RAM that is "recognized" by 32-bit OS...
Anyway, thanks for all your help! Hopefully this thread might help other guys with similar problems.