SIM900 random shutdowns
Cool, an UBEC is indeed a smart choice for powering a SIM900.
I could get away with a small switching power supply and just putting in a big capacitor (since the sim900 only draws high current in very short peaks).

But, the limiting factor in this case would be the 6AA batteries. Your UBEC may be able to supply 5A's but those AA's may not.

Also, look up "transient response" (related) according to the related article, you can have an 20A power supply, but still have the voltage drop out when the sim900 starts to transmit. This is because your supply needs to be able to deliver the required 2A in a very short period of time, so basically if your supply can't go from 100ma to 2A fast enough, it'll dip.

I suggest finding a more optimal source as AA's and adding a "big ass capacitor" close to the sim900's input.