+1 on the Hammer products. Try Sustained Energy with a little Hammer Gel in it, so it comes out tasting OK and about 750 cal./bottle, enough for 3 hours. Plain water in the other bottle. For the century, carry a baggie with powder in it to make more, and a flask of HG. Also get some Endurolytes. Try 1-2 Endurolytes every hour, or enough to create some thirst. Put them in a coin purse shoved up your shorts leg if it's not raining. Well worth a try. If it doesn't help or fix your problem, don't buy more.
Conditioning might also be part of it. Try to maintain 150 miles/week. Also, practice getting out of the saddle for 1 minute in every 10, or every time you come off the front of a paceline. Stretches the legs. Practice eating a little SE mix and drinking a little water every 15 minutes. Watching the time adds to your situational awareness, so you don't drift off into funk and mess up your hydration, etc.