Macbook Pro intermittently hangs at grey screen during startup.

Ok everyone, this 15" Macbook Pro (2010) hangs intermittently during startup. After the Apple appears, at the gray screen.

TechToolPro 7 reports everything is OK with memory, hard drive, etc during a full test.

Diskwarrior says everything is OK with the hard drive

Disk First Aid reports no errors.

I've reset the PRAM several times (Incidentally, it usually starts up just fine right after a PRAM zap, and hangs other times)

Any ideas what it could be? 4GB RAM, running OSX 10.6.8 with all known patches applied.

Thank you in advance,

James