Probably just a coincidence that your 360 is starting to malfunction after updating to NXE.
Well I'm pretty sure the nice - if barely understandable - Eastern European chap I spoke to on the phone today confirmed that it's a known issue with NXE. I did have to make him say everything twice though.
You can't revert back to the old dashboard.
Well that makes me feel a bit better, since the freezing is now happening far too quickly for me to be able to clear the cache. Or backup any files. On the plus side, it still happens without the hard drive connected, so the hard drive is probably fine.