Default [Q] Touch screen does not respond

Hello there. 3 days ago i bought used Galaxy S2. Everything was good untill now. Ive got an sms message and i wanted to check it soni unlocked my screen and suddenly my screen went crazy. It starts to "touch himself"(that sounds bad) and i couldn't do anything. Only locking and unlocking the screen was a solution. Now it happens even more often. I reinstalled android already, wiped data and cache and still nothing. Probably the solution is to change digitalizer or something but i just want to know: What is causing this issue? Thanks for any helpful ideas and tips. Cheers