Hmmm, sounds like it could be electrolytes then. Be sure to keep the hydration at 500-750ml/hr right from the start. Some previous threads on the subject:

Why do I cramp so much?
Combating CRAMPS; running out of ideas!! PLEASE HELP! (before & after report on the solution)

Sports-drinks are woefully inadequate at supplying the necessary electrolytes and glucose to replenish what you burn up on a ride. I would mix my own Gatorade in a double-concentration mix in order to get the required 250cal/hr and sufficient electrolytes in a 600ml bottle (pre-made Gatorade would require you to drink way too much volume in order to get sufficient nutrients). You can also take electrolyte tablets like eCaps, e-load, Endurolytes or similar products.