iPhone 4S screen does not respond

I've had my iPhone 4S for almost two years now and have had regular touchscreen problems but they have become an everyday problem now. When I try to make an action (like enter my passcode) the screen would lag, simply not sense the touch, hold a touch, or hold an action (touch) but then retract that action. Everytime it got like this, I would restart the phone. But now that RARELY works. It's like the phone only works when it wants to. I'm almost certain I've tried everything but I hope there's something I haven't. I've also tried charging it differenly, leaving 4GB of free space (thiniking it could just be slow), etc. Can anyone help me out? Or at least tell me a reason for this? Thanks

EDIT: I just noticed a tiny black line on the upper left corner of the screen. I turned the phone off, back on again and it was gone. WHYYY... O.o

I also may need to add that it has turned off by itself in the past (when it was almost fully charged). The only way to turn it back on was to restart it