iPhone 4S losing data connection while still showing 3G/EDGE symbol and appropriate reception

Same problem here ....Using a UK iphone 4s factory unlocked in mumbai with tata docomo sim card ..... Edge is working fine but suddenly it stops working ... I can see the full signal bar with the edge logo still it wont work then i need to on off airplane mode to make it start again .... Happens 3-4 times a day ... No idea whats causing this problem ...