I have a Samsung GTI 5500 smartphone which has developed a habit of becoming unresponsive when left on standby. To bring it back to life I have to take out the battery, put it back in and then restart the phone. It does it 2-3 times a day but not when it's in airplane mode (which it's in overnight).
Any ideas? If it helps, the only app I tend to use is Whatsapp, using wifi.