iMac grey screen at startup after clean install

One of the iMacs at work suddenly stopped completing its startup process. I starts, chimes, pinwheel spins, then gets stuck on the grey apple on a grey screen.

We've tried all the usual: disconnect everything; reset NVRAM; reset SCM, boot into utilities and repair permissions & verify disk.

It boots okay in safe mode. No 3rd party things in login items.

We did a system restore; didn't help.

This machine had its optical drive removed & SSD installed, leaving its original volume empty. We used a flash drive and installed a fresh copy of Mavericks onto the empty HD, and were sure to choose it as the startup volume. We expected the Apple welcome/setup screens, but got the same grey apple on a grey screen. Safe Boot was able to complete the new account setup, but on restart it want back to grey.

What gives??

Thanks in advance for any thoughts on this.

It's a 3.06 ghz, 24" Core 2 Duo. 8 gb ram. ATI Radeon HD 4850.