I have iphone 3g problem when i enable 3g signal is perfect but if i disable 3g i cant get 2g signal. any advice? thanks in advance.

Update #1: I opened my iPhone 3G to look for any modifications inside but there were no such things. When i closed it i started to receive gsm signal from time to time, but the problem is that i cant receive calls and if I try to call someone signal drops and "no service" shows. like there is no enough power.

Update #2: I can't get gsm signal again... i don't know what happend... woke up last morning there was no service. I'll try last update (3.1) to see what will happen.

Update #3: First of all my gsm signal is back with standard problem of droping from time to time, but this time i can make/receive calls and all other stuff. Anyway signal drops became very iritating so last night i updated to 3.1. First of all i noticed better batery life, but on other hand i cant get now 3G/2G signals. After reseting network settings 3G signal is back, but 2G signal keeps droping so i can presume that gsm chip on motherboard or maybe battery itself is dead.

Question: Because I opened my iphone 3g and closed it i could receive better network reception. I'm getting confused and i'm asking could be that battery is failing... or indeed i need to replace my motherboard?

Thanks in advance