There are a few possible causes for this behavior:
- Bad/defective RAM sticks
- Bad/Defective RAM sockets/motherboard
- Insufficient power being supplied to the RAM slots
- Mis-matched RAM specs/timings (you seem to have that covered)
You can test the individual RAM sticks using Memtest86+ and installing one stick at a time in Slot #1 to run the test. (be sure to mark/number the sticks if they are identical)
You can test the slots by populating the #3 and running Memtest, then #4.
You can test power issues by going into the BIOS and increasing the voltage for the RAM in small increments and testing. Do not exceed any motherboard limit mentioned in your manual.
Hope that helps.