Rode a century yesterday and at the 3 hour window I hit the same thing. Rode a good pace the first 50 miles then power just fell off. Rode very slowly to the next stop, went to the bathroom and my hydration is off. I consumed 2 bottles took 10 minutes and then hopped on the bike and I was back to normal performance. Felt strong. So my issue is hydration related which has become more pronounced as I am riding significantly harder than last year. I had been drinking the 1 bottle an hour when riding but obviously that is just not enough. I also need to be much smarter about stopping and hydrating. I was with a group towards the end of the ride and skipped one rest stop which left me short at the end. My eating was more normal with something every hour and a banana and snack at a rest stop. Looking forward to next week when I'm hitting a 100 mile 9500 feet and going to make sure I am well hydrated and my bottles are always filled.