As I mentioned in my first response that this is commonly due to local storage, that's only because any time I resolved the problem we swapped out local storage.
In one case they were using a non-standard USB key (e.g. they got it at Best Buy/Frys/etc) and was not certified for the host by the hardware vendor. Replacing with a fast certified USB key resolved the problem.
The other case it was an ESXi embedded host where the internal CF Card was having issues. It was faster to re-install with boot from SAN to at least prove it would work, which it did. This technically excludes the ESXi embedded image as a problem, but generally pointed at the install/CF card as the issue.