I would suggest heat is killing the inverter, my background is high integrity engineering in military equipment. Inverters are designed to self-protect from heat effects, and newer inverters are better at that thean older models, BUT, heat still shortens the life of electronics. On most of my solar installs the inverter is either in something cooler like a garage, or mounted on an external wall in the shade - not had to change an inverter so far in 8 years. I would add some simple roof vents to improve attic air circulation and some vents in the eaves so you create a natural air flow. Option 2 is move the inverter out of attic. A cooler inverter will work better in the hotter months, so improving FiT income and supplying more power to your home.