Two remarks: Both of my serial 2.2-GW's stay at "startup complete" and are working as expected (seems to have changed somewhen in the development of 2.0). Trying to set them to "connect" causes a reboot ending again in "startup complete". So this is not really a fault, just let them state "startup complete" and don't worry too much about that. If the Arduino really is spontaneously rebooting, this could also be due to a fault in the Arduino design (Test PION floating). But as you described the "problem" shows up also with the adapter/pro mini version, this seems not to be the case here.