Hi,
Sparkfun says:
This unit has an onboard 3.3V regulator...On-board 3.3V LDO Regulator (3.3 to 7V supply allowed)
Sounds like you need to power it from 5V..
Other: What software are you using??
The RF Power Level can be set. Start with lowest. The possibilities are: RF24_PA_MIN, RF24_PA_LOW, RF24_PA_HIGH and RF24_PA_MAX Keep the two units separated by several feet.