This happened to me on my last 400k at about mile 150. Too much gatorade (and sugar) will upset my stomach, at which point I switch to plain water. What actually worked for me was drinking at v8 (at the suggestion of one of my riding buddies.). I started to feel much better within 30 minutes and finished the rest of the ride strong. On long rides I like to get real food sometime around halfway through - my favorite is a 12" spicy Italian subway sandwich. I'll eat 1/2 & stuff the other half in my pocket and nibble as I go.