Just an update on the crashes and hangs I was seeing. I think it's been resolved.
As I mentioned, I was seeing this in one of the crash logs:
Code:
325.831> [eventData] EPG Cache is corrupt (eventData::~eventData), you should restart Enigma!
This error which lead to a crash was on the client receiver where fallback tuner and EPG import is enabled. Turns out the egp.dat on the server receiver had gotten corrupt (was only 110Kb instead of 18Mb) and everytime the client receiver booted, it FTP'ed the epg.dat from the server receiver (getting a corrupt copy every time).

So it appears to be totally unrelated to the 8GB USB stick and 50/60Hz issues.

But it does explain why it started happening when I upgraded from Hades018 to Vix 5.0.11 because the fallback tuner + EPG import wasn't possible on Hades 018 (and I guess the epg.dat was corrupt either)

Final question - should I schedule a cronjob that deletes epg.dat on the server receiver nights before CrossEPG runs? It might help avoid this in future