samsung galaxy note gt-7000 problem

hi all im new to this site and the note firmware rooting roms etc so please bear with me as im still trying to understand what they all mean i bought the note off fleabay last week my problem is when you are doing somthing all of a sudden the screen will start going back as though the back button is being pressed but it is not when i got it i think it was on 4.02 firmware and in safe mode and was doing it all the time so updated on phone to 4.12 and kept doing it but ive noticed the more apps ive installed the less its done it
my baseband version is N7000XXLSO
kernal 3.0.31-906407
build number JZ054K.N7000XXLSZ
hope this helps and anyhelp appreciated thanks