Is it a coincidence?

Hot weather in March - connection dropping or very slow. Hot weather in May - same story. I've had to turn off my modem and router this pm as they were very hot and when cooled down I was back on 37Mbps again. Could be heat in the cabinet outside I suppose.

How is your modem mounted? Flat or or a vertical surface?

I've screwed it to the wall vertically.

Does it have 3B on the sticker on the back ?

My modem is in direct sunlight and works fine. It says 3b in big letters on the bottom. Never been turned off

The engineer that installed it said previous versions should be wall mounted to keep them cool.
Hope this helps

Have just measured the temperature of te room where my modem, router, switch 2xWAPs and more are located. 34^oC and that is at 22:00, mid afternoon when outside temperatures were higher, it may have reached a degree or two higher. The modem has perform fine all day.