If it turns out that all of the dimms are okay, here are the probable causes that come to mind:
1. Motherboard isn't setting proper timings and/or voltage for the number of dimms you have installed. Try setting it manually to 3-4-4-8 and 2.7v. If that works, go back to rated timings and use as little voltage as needed to make it work. Don't depend on the BIOS programming to read and execute the SPD timings for all configurations... there have been BIOS bugs (aka sloppy programming) which caused incorrect settings (not necessarily yours, just mentioning that it has happened). You must run 2T command rate, with more than two dimms, on most or all motherboards. If you're not familiar with identifying and adjusting these settings, the time has come to learn.
2. Either the motherboard or the power supply can't supply the amperage needed to all the dimm slots in use.
3. The chips used on the newer dimms don't play well with the older ones. It does happen. Lower frequencies and/or higher latencies might fix that.
4. Heat is building up in the area around the dimm slots.