I had the same problem after replacing battery and menu button on my iphone 4. It worked fine for 2 days then had the same problem even when you try to restart it.
Everything went well. Took me 3hours to tear it apart and back together. the only issue im having is that sometimes my screen won't turn on even thought the phone is on and im receiving text msg and calls. So each time i have to restart my phone by pressing the switch off and menu button(im glad i change it or else i wont be able to restart) anyone knows why this is happening? Some help please