Did you install any Market apps on the replacement phone ?
Very well could be an incompatability with an app and GB..android 2.3.
If yes, try starting the phone in 'safe' mode and see if it works OK
or uninstall all market app and use phone for a couple days to see
if it now works OK.
Then reinstall one app at a time, use phone awhile before installing next app.
When phone breaks, thats the offending app.