deachus, first of all, if you want to have your rig rock stable you'll have to pass all tests as there is no absolute and perfect stability checking program. From my experience successful (and long enough) testing with Prime/Orthos, Linpack and Memtest is usually enough to prove stability. Still, neither of these programs does test Northbridge well enough, so some failures in 3d (not necessarily 3dmark though) may occur due to NB instability even after successfull Prime-Linpack-Memtest runs.

As for Linpack, 20 tests just isn't enough in most cases. Why do people test their rigs with Prime for 5, 8 or even more hours but keep thinking that 20 Linpack runs mean stable system (when it takes less than half an hour)? Linpack is really great in determining CPU instability, but you should consider running it at least 50 times at maximum available memory if you want to call it stable.

What's more important, Linpack is sensitive to vcore only, wrong vtt/gtl settings and/or slightly unstable RAM won't cause errors in Linpack (at least during 20 runs). But will most likely cause them in Prime. If Memtest and Linpack pass ok, but Prime still fails, I'd try to tweak vtt & gtl settings.