NOTE!! Most * problems with intermittent operation are because of electrical noise on the 3.3V Power supply. The MEGA is more of a problem with this. Solution: ADD bypass capacitors across GND and 3.3V ON the radio modules. One user said, "Just Solder a 100nF ceramic cap across the gnd and 3.3v pins direct on the nrf24l01+ modules!" Some have used a 1uF to 10uF capacitor.
NOTE: Pin 8 IRQ is Unused by most software, but the RF24 library has an example that utilizes it.