screen turning off while in use

I have a MyTouch 4G Slide of which I replaced the flex cable that connects the logic board to the screen and the touch screen/digitizer. all seems to be working fine, except for one thing: sometimes the screen goes black. for example, when my client goes to reply to a text, the screen may turn off. the phone doesn't turn off though. He's also concerned about his battery life. This phone has a sleep mode but you have to actually make it go into sleep mode or it happens after a long time of inactivity. the screen is turning off while in use.

Has anyone ever heard of this happening? Could a bad battery cause a sleep mode to occur inadvertently? ANY and all ideas/suggestions would be greatly appreciated.