RB751U-2HnD reboots acidentialy

Hello,

I have issue with RB751U-2HnD, it restarts when there is some moderate not even high load on wireless interface.
To clarify, if there are few clients for example 3 clients, with standard internet activities like web browsing e-mail checking everything works well, but when someone starts to copy file to server (connected to ethernet port, bridged with wireless) router reboots itself. And if client restarts activity it reboots again and again, sometimes it takes few seconds sometimes bit longer, bet it reboots anyway.

I can’t exactly identify when it started, but it looks like after upgrade to 5.19 and now 5.20 same problem, before was 5.16.
Is it safe to downgrade os?

other possibility is it started after adding external antenna, but it was bit longer ago, maybe just didn’t notice such problem.
Have played around with antenna settings, but haven’t find solution.
Current settings:

/interface wireless print
Flags: X - disabled, R - running 
 0  R name="wlan1" mtu=1500 mac-address=00:0C:42:FB:A0:F9 arp=enabled 
      interface-type=Atheros 11N mode=ap-bridge ssid="<my ssid>" frequency=2427 
      band=2ghz-b/g/n channel-width=20/40mhz-ht-above scan-list=default 
      wireless-protocol=unspecified antenna-mode=ant-b wds-mode=disabled 
      wds-default-bridge=none wds-ignore-ssid=no bridge-mode=enabled 
      default-authentication=yes default-forwarding=yes default-ap-tx-limit=0 
      default-client-tx-limit=0 hide-ssid=no security-profile=<my profile>
      compression=no

Have anybody ideas what can I do/try to solve such annoying problem?

I have seen this happen several times when an incorrect power supply was used - reboot under wireless load

Good point, thanks, will have to find another adapter and try what will happen.
It would be a bit strange, as I’m using native PS, but of course it is worse to try.