Powering SIM800L

Hi,

I have a GSM SIM800L EVB. I use pololu D24V60F5 step down regulator for powering it.

It was working fine for the first hour, but now it restarts always when try to send gprs data.
I have put a 470uf capacitor just before the input power of the gprs module. Still restarts.

What is the problem?

Thanks!