Screen protector removal worked for me as well after I tried everything else. I guess the core issue here is that n900's own resistive film is very thin to achieve great response and that any protector is made of another material. Every time there is a temprature change any material changes its dimensions a bit. since the protector is thcker - it forces device's film to change them with it, which resilts in sporaodic touchscreen issues.
I stopped using screen protector and haven't had a single touchscreen issue since that.