appropriate power source for sim900 or neoway590 modules

Hello guys. i'm lost again

i made several projects using arduino gsm shield with sim900 module (

I used LM256 module to power shield via external jack. For one project (one shiled) everything works great. I ordered 5 more shields, made same circuit with LM5256 module, and modems (all 5 of them) reboots while trying to register to network!

It seems like power related issue. I tried to connect LM module to 12v 2a power supply, to 12v lion battery - result the same, although i have exactly same shield working with same configuration.

Guys, help me, i'm going nuts over this one :(((

UPDATE: i've user 12v 5amp power source, dropped down voltage to 5v using LM5256 and results are the same.

Maybe someone could advise how to get most stable 5v output, or how to apply decoupling caps on this gms shield?