touchscreen stops working intermittently

I have an iPhone 3gs 16gig and the I broke the screen, the digitizer worked for a while so I just kept using it with the cracks on it. after the digitizer stopped working I did a full backup and went in to a local shop and had the screen and digitizer replaced. I then came home and did a restore and that gave me ios5.1. The phone worked great for a day then the touch screen started to stop working after about 10-30 seconds of use. I cant do anything with the phone without having to lock the screen and unlock it (sometimes that works, and sometimes I have to do that a few times because the touch screen doesn't work to unlock the phone) to continue using my phone. I am thinking it is because I upgraded to ios5.1, or is it a bad digitizer? can anyone help? or does anyone have the same problem? thank you.