@rvendrame said:
Once I had similar problems, a 4.7nF cap and gw.wait(100) after each transmit did the trick for me... Hope it helps somehow.
I also got to this solution when experienced the same 'st:fail' issue. The arduino's delay wont help, because it wont receive anything from radio during the delay.
As i got mutch less or no error with ack = false, I thing its linked to receiving some ACK packets. I have no deeper information or debugging possibilities at the moment.