Gigabit degrades to about 33Mbps when it rains heavily

I've been installed for a couple of weeks now; a tech came out to help me do my self-install and rewired our messy junction box. After install, I was typically getting 800Mbps initially. 

Then rain hit and I noticed my speed dropped dramatically to around 30Mbps. I restarted my modem after the rain and got back to ~800Mbps. This sequence has repeated each time it rains. Twice today.

What approach should I take to resolve this? I have a hunch it's not just my house, and that it's degrading in our neighborhood but have no way to confirm.