375 x 6 gives the same errors as 375 with max mult? Ok then, it's probably VTT. Even though it shouldn't be at that kind of FSB with C0 chips...
Maybe (just maybe) your prime version is ed up? You said you are using 64bit prime.. don't. Use the normal 32bit V25.5 or 25.6 and retest.
If the problem persists, you will need to run FSB 370 at max multiplier to remain stable