Why does the iPhone keep going into the emergency call screen?

I've been working on a customer's GSM iPhone 4. They came in wanting the home button fixed. Upon taking the phone apart, I noticed that plug that the home button switch plugs into had been water damaged, due to the calcium build up on it and the button switch cable. I swapped out both parts and that fixed the home button issue, but then the screen kept shutting off and going into emergency call screen, AND the temperature screen, telling me that the phone was too hot and had to cool down before using. It obviously wasn't hot. On the upper left part of the screen, the phone would say that it'd be searching for service, then would find the at&t service, and then would say no service -- and it would continue to cycle through this until the battery died, if I let it.

I've tried a different battery. I've tried different charge ports. I removed the battery and let it sit over night. I checked all the resistors and plugs to see if I had accidentally snapped one off from taking the bloody thing apart so many times -- but all are intact. I've replaced the antenna. And finally, i restored the phone. It does all the same things that it did before, but it's not telling me that the device is too hot, and it's turning itself off and on.

Just wondering if anyone has an answer. The only thing I can think of doing is asking the customer if it's still under warranty so that they can send it in and get a new phone.