it is not problem with power supply, because i am using laboratory power supply that can provide more than 2A
It is not guarantee you are supplying GSM module the right way. sim800l needs power supply voltage 3.4-4.4V recommended 4V. Burst may produce problems because i can pull up to 2A current and if you have long wires it may produce voltage drop across it and GSM module will reset. It is good practice to solder electrolytic capacitor as close to the module as possible. I use 1000uF 6.3V cap and it helps. It is better to use a couple of lower capacity low ESR type capacitors.
On this link you'll find the pdf document http://wiki.seeedstudio.com/images/4/46/SIM800L_Hardware_Design_V1.00.pdf
Look at page 21 and couple next pages for sim800l power supply recommendations.
Best regards...