Windows 7: Custom .wim sometimes works, sometimes 0x80070570, help figuring why?

Hi, as the title said I'm using a custom install.wim with Windows 7 x64; it's working sometimes and not others, or on some machines not others (can't find a pattern at the moment). Summary of what I've done and checked:

Windows 7 allows the use of sysprep/dism to create a custom .wim file. Replace normal .wim with this, and in theory --> custom installer DVD that works exactly as usual Windows 7 installer, but installs your system pretty almost as imaged. Invaluable for ad-hoc reinstall of a system with stuff all in place, and saves hours or (for a detailed custom install) most of a day each time.

So taking it from there:

I have a USB key formatted as bootable NTFS with the entire Windows 7 DVD copied to it, except for the custom install.wim replacing the usual install.wim. I've used this USB key without modification to install at least 3 different machines (file server, laptop and another) so I know it installs without problems. It does have some issues, for example repair or recovery don't seem to work as completely they do on the original, can't figure why not, but install itself works.

Today I tried reinstalling one of these. I've used this installer on it before so I know it works fine. Error 0x80070570. Check RAM - windows ram checker and memtest86+, both A-OK. Check custom .wim (and all of \sources) SHA1 against the SHA1 at the time of creation - all correct (not a corruption error AFAICS). Replace custom .wim by original .wim, install works, apparently confirming the rest of the installer files and RAM are ok. I set about debugging the failed install of the custom .wim, and start by booting the new install, then running the custom install off hard drive from within Windows instead of USB boot, in case USB is somehow an issue. I get the same error on the custom .wim, but now also 0x80070241 on the standard .wim.

Most fixes for these issues seem to assert it's a file corruption error or memory error. I've tested AFAIK the most common issues (hardware compatibility, RAM fault, installer .wim corruption, installer medium readability), and the same installer has worked on other machines. So where to go from here to get it working correctly or figure where it's failing?