Weak signal after screen replacement

Hey,

I replaced my screen a few months ago and it's been playing up ever since. But I've finally got round to posting here as it's starting to really bug me.

When the phone has 3G signal everything is fine, I can make calls etc.. without a problem.

When I only have EDGE or WAP, the signal is terrible, for example if you make a phone call, the sound will cut out every few seconds, it almost impossible to talk to people. I just have to tell them to text me.

Any ideas what could be up with the phone, like I said I replaced the screen, but I have done a few before, and they've all been fine :(

Thank you in advanced,

Aidan