I think I may have a battery issue..

I ran the battery down over the weekend and the device turned itself off. After I plugged it in the charger and turned it back on, the lock/resume issue started happening again. Removing the battery didn't work this time.

I then charged it fully (via USB) and removed the battery for 5 mins and it started working again... but after an hour or so on screen lock, it won't resume.

I'm thinking there is an issue with my battery - current probably drops to a point the phone thinks it is out of juice. I might try buy a second battery and see if that fixes this issue.