I´m having the same problem... I get to receive around 8 messages from my end device and then, the gateway stop receiving messages at all. I have a 2A power source for the RPi3 and a 2A power source for the RAK831.
After a while of not receiving messages, it suddenly starts working again... and then, another 6 - 10 messages and stops again. could this be a packet_forwarder issue and not an electric one?
PD: I'm also having -120 RSSI and my device is 4 meters away only.
Thanks in advance!