Home button: erratic behavior

Dear iFixit,

I searched the old posts but have not found a solution.

The home button on my iPhone 3G after it went from OS 2.1 to 3 (and now at 4.2) has some very strange behavior.

Every now and then smashing it works perfectly, while other times you have to press it very very strong to be able to operate. I do not think a hardware problem (because as I said sometimes it works fine) but something of a 3d generation, the problem is more noticeable when you use applications 'of a certain weight' and I can return on home screen only if I press very strong (so strong that the LCD screen floats...)

Do you have any suggestions to help me? I'm going crazy and use it so it's very frustrating.

I just did a restore when I switched from OS 3.1 to OS 4.2.

Thanks.

Vincenzo