My iphone (1st gen I will discuss first) would open the application and it would shut down or totally freeze up.By holding down the home key for 5 seconds would un freeze the phone but not fix the situation.
Well I went through the same occurrences with my 3G iphone.
The solution was uninstall applications and re install them again.
The 3G still was not acting up to par , so I wiped it clean and re-downloaded the firmware , this helped allot.Everything that I have on my mac I can put back on the iPhone/iPhones.