Signal work 5 minutes, then it's lost!

Hi,

I bought a iPhone from a Guy who dropped it on the driveway. It had a Otterbox Defender on it, so, the phone was still ok. The frame was bent, the front and back screen were a disaster. I bought a new frame on ebay, and I rebuilt it completely. It works fine now, but the only problem is that I am not able to keep the signal. I get a good connextion to the network for a few minutes and then, it drops, and it writes ; searching... and after that : No network.

I did my homework, I readed thread about problem with gsm signal on iPhone 4, and I saw there was some tips saying to wash it, so I did a teardown again and washed it carefully. I assembled it again, and I had the signal, but again... only for a few minutes ! I lost signal and then, 20 minutes later, got it back ! So now... I really don't know what to do.

any suggestions ?

Thanks !