What resolved this for me:
Settings > Wireless & Networks > Mobile Networks > Enable Always-on mobile data > Make sure this is checked and enabled.
I read a 'Battery saving' tip to disable this, i'm guessing some people did this. I did it and it does save a lot of battery but it causes the UI to lock up and freeze as well as intermitent reboots with constant connection failures (manual reconnect needed constantly)
Once enabled again reboots stopped.. my big battery saver was this from another thread-
"
press HOME, and bring up the dialer. Dial *#*#4636#*#* (Do not press "Call"). After a second, it should bring up the "Testing" menu.
Go into Phone Information, press MENU, and tap "select radio band." You should experience a Force Close, don't panic, this was supposed to happen. Scrolling down shows that the preferred network type is GSM auto (prl).
What this means is that even though you are connected to a CDMA network, your phone is trying to find a GSM network, preventing the radio from sleeping, and causing excessive battery drain.
Remedy this by tapping GSM auto (prl), and changing it to CDMA auto (prl).
Now your done, your TWS should drop dramatically, and you should experience much better battery life!"