I used to get that symptom on my 720.

Rather than investigate, I found two totally unscientific solutions:

1. Boot several times until it magically worked :-)

2. Only use one flash card at a time.

Whenever I had a PCMCIA flash and a CF flash together, the boot would hang
about 4 in 5 attempts. With either of the flash cards alone, the boot works
perfectly every time.

Of course, I have the lithium battery removed to make rebooting easy.