I have some problems with my GS
1- when someone calls, another name from contact appears on the screen instead of the caller, really annoying
2- The display turns on/off on its own randomly, I turn off the phone and it turns on every few seconds ( display light wakes me up at night)
3- It takes 3-4 seconds to open contacts or other applications ( my iphone never done that, I switched to SG because I hate dictators)
4- I live in the US, the phone I have is from overseas, any idea how to make 3G works on ATT)
I'm not sure if this is accurate but all your problems may be the fact that you have an international version. I would do a factory reset of the phone and start over. That should correct all your problems unless the international version is not compatible with US carriers. To do the reset go into settings>privacy>factory data reset. Try turning the phone on and off 1st. Hope this helps.
By the way, your #2 might be your phone getting an email or some other notification in the middle of the night. #3, a phone taking 3-4 seconds to open an app is not an issue. Some of my apps take that long but others are quicker. There could be many other issues that could be slowing your phone down. Also, I wanted to mention to check the phone numbers in your contacts to make sure they are entered correctly in the contacts that are popping up in error.