Dual core machines block frequently when running vanilla q3, usually on startup.

I've located this to win_glimp.c, line 1637.

Heres the execution path that blocks:

Thread 1 (T1) is running GLimp_RendererSleep:
Thread 2 (T2) is running GLimp_WakeRenderer:

T1 resets renderActiveEvent
T2 sets the renderCommandsEvent event
T1 waits on the renderCommandsEvent event, passing through
T1 resets the renderCommandsEvent event
T1 resets renderActiveEvent event

T1 then executes GLimp_RendererSleep again, resetting renderActiveEvent
T2 waits on renderActiveEvent, which is unset.

Because T1 reset renderCommandsEvent earlier, it blocks on waiting for it.  T2 hasnt set it, because its already set it and is waiting on renderActiveEvent, which T1 already unset.

This can be fixed by moving the line:
ResetEvent( renderActiveEvent );
from line 1637 to just after WaitForSingleObject( renderCommandsEvent, INFINITE ); on line 1642.