SIM808 module restarting repeatedly

I have bought SIM808 module, everything working fine. All AT commands are giving expected response, i'm accessing it through hyperterminal. My problem is that whenever i make or receive a call or SMS , GSM gets restarting.(I can able to make/receive call, getting ring/alert for small period of time and then interrupt). But there is no Under or Lower voltage power down or warning.
I have tried with DC power supply of 5V/500mA , 5V/1A and 5V/2A. What might be the reason ? Please help.
extra info : It has one LM2576S voltage regulator in-built.