I've reinstalled OS X Mavericks from scratch twice on my (usually headless) Mac Mini, but it keeps locking up after ten or fifteen minutes of use - even just Safari browsing. I use Screen Sharing with the firewall off (for now - to rule that out as a cause of the problem).
I connected an HDTV to see what was going on when the Mini locked up and I saw this:
...which doesn't look good.
I've run a full 'memtest' in single-user mode, twice, and that says it's OK. I'll try AHT when I get the chance, but does anyone have any advice or suggestions as to the cause?