The work I do has forced me to mix and match RAM in my PC to temporarily increase capacity. The following is what was being used before any changes. i5 4690K 4200MHz VCore = 1.1V VCache = 1.1V VInput = 1.9V. All other CPU voltages left to Auto in BIOS. Core, cache, and input voltages were all set to fixed voltage. ASRock Z97 Extreme 4 GSkill DDR3-2133 8GB (2x4GB) overclocked to 2400. VDRAM (overclocked) = 1.65V. Timings (overclocked) = 11-13-13-31 This setup has been the daily usage setup since 2014. No stability issues in any form. I added the following kit. GSkill DDR3-1600 16GB (2x8GB). Rated for VDRAM = 1.5V. Timings = 10-12-12-30. Then I configured the combined RAM (24GB) to run at DDR3-1600 VDRAM = 1.5V My workload is RAM intensive as it requires reading multiple gigabytes into RAM, doing calculations (single thread) on the data in RAM, then writing the result back to disk. This was set to happen 20 times. This workload was sufficient to send me back to BIOS (by the 5th or 6th of 20 iterations) and only occurred when all 4 memory slots were populated. I expect to find instability in mis-matched RAM kits when the RAM slots fully populated (maybe I didn't get the best silicon lottery draw of IMC). For reference the workload (MATLAB2019) was run on Debian Linux 4.19 kernel. After doing some reading on the specifics of Devils Canyon (a little different than Haswell) in how Intel set up all the voltage names and CPU parts, I made the following adjustments in BIOS. All of these were done with the RAM at stock DDR3-1600 VDRAM = 1.5V Timings = 10-12-12-30. CPU core clock = 4000MHz (decreased. stock is 3500 with 3900 turbo) VCore = 1.1V (CPU core voltage. no change) VCache = 1.1V ("Last Level Cache"/Ring voltage. no change) VInput = 1.9V (Voltage Regulator Input voltage. no change) VSA = 1.05V (System Sgent voltage increased from stock) VIOA = 1.1V (Analog Input/Output voltage. increased from stock) VIOD = 1.1V (Digital Input/Output voltage. increased from stock) From what I read the system agent and IOD/A voltages all influence the IMC so raising those a little (100 to 200mV more than stock) should help stability under load. Despite all of these changes however, my workload would still send me back to BIOS. I couldn't reproduce the behavior in Prime95 (custom high RAM usage test) and OCCT (Linpack test) on Windows 7. Presently I'm using these voltages with only the 16GB kit at its stock settings and everything is fine. Has anyone had a similar experience with Haswell era parts? I was considering buying more RAM before DDR3 becomes impossible to find. I wouldn't be considering a DDR3-1600 kit however and would likely aim for a higher speed 2x8GB 2400 or 2133 kit. Could the cause of my RAM issues be the mismatching sticks? Would using more similar sticks (my 2x4GB 2133 kit along with a 2x4GB 2400 kit) lead to better results? Last but not least, is this a software issue?