It seems that 4.0.6 has problems with memory management.
I understand (and this is completely normal) that 512MB model has only ~200MB free ram after boot. But running any resource heavy app- like Firefox (both official and Aurora) or some games (but not all- some take care of freeing memory by themselves when exited)- drops free ram under 80 or sometimes under 50 MB and not always memory is reclaimed after stopping the app in Settings->Apps->Running.
And BTW could you add to Recovery
-confirmations for all format/wipe operations
-wiping dalvik-cache
please?
Yes there is a definite issue where memory isn't being freed. I have resorted to using a 'fast reboot' app and keeping a close eye on unnecessary processes.
Otherwise the device will often freeze/restart.