Did you replace the power brick for a decent Samsung/Apple one (delivering 2A or up)? I’d replace the cable as well.
If so, and when the “full software download” didn’t help, I’d like to recommend contacting homey.app/support.
Can I assume you know wich 2.4GHz wifi channel is used (don’t use auto-channel)?
Maybe your close neighbour got a new wifi system for Xmas, which ruins your zigbee signal?
