Have you done a full factory reset or even a full reflash via uboot? That would be the first place I’d start.
Use this firmware:
http://www.gl-inet.com/firmware/mt300a/v1/openwrt-mt300a-2.264.bin
with this procedure to flash:
If the issue continues with a clean flash and seems to persist across multiple different power supplies, my guess would be hardware issues. If, after a clean flash, it works, I’d stress test it by using it for a few days in a somewhat default/vanilla configuration before configuring OpenVPN and the like to rule out configuration troubles for the instability.