Why won't my Xperia touchscreen work?

Nope, it’s not the screen. All sites, including Sony’s official troubleshooting, say to clean the screen (or even replace it). But I’m pretty sure it’s Android-related, so I’m hoping any wiz-kids out there might know what it is, even if they don’t have the same model phone.

Mine is a Sony Xperia Sole, latest Android version, bought it back in June and it’s generally in good condition (despite the odd bump).

Last month it stopped responding all of a sudden. Not completely, but you have to slide or tap several times to get it right, you tap somewhere and it thinks you tapped somewhere else, etc. Typing messages is impossible, even answering the phone takes anywhere from 1 to 20 tries.

The funny thing is that at some point it worked perfect for a day, then stopped again. Then 9 days ago it worked perfect again until last Wednesday, and hasn’t worked properly since.

I’ve tried cleaning the screen, but I’m sure it’s not related.
I’ve restarted several times. Sometimes, it works for a few seconds, and then, as soon as the apps load, it has the same problem again.
I’ve uninstalled practically everything, stripping it to the bare essentials. Still the same problem.
I’ve even tried unmounting the memory stick and SIM card.
And I’ve changed the charger, in case it was the voltage.

The only hint I have is that last time it got fixed I had just taken a few pictures of my daughter. Maybe after you use the camera and hit “home” or “back” (I’ve tried both), the camera is still running and using up system resources? It’s the only explanation I can find so far. It’s acting just like a computer which has too many programmes running and the RAM gets stuck so they freeze and the mouse has slow response. That’s why I think it’s software-related and not the touchscreen.

I need a human to help me fix this, because all the sites I’ve found say the same thing: clean the screen with a cloth or replace it if it’s scratched. I repeat: it’s NOT the screen!