Ducati Marlboro's Casey Stoner arrived for the Red Bull U.S. Grand Prix still suffering the stomach ailment that cost him any chance of competing for the victory in both Barcelona and Assen.The problems first appeared in Barcelona three weeks ago. Stoner was visibly laboring after finishing well off the pace in third. When the same thing happened two weeks later in Assen, there was further cause for concern. Stoner needed an intra-venous drip and wasn't able to attend the post-race news conference.This weekend, as he returns to the site of his epic 2008 battle with Fiat Yamaha's Valentino Rossi at Mazda Raceway Laguna Seca, his health will be monitored by doctors of the Clinica Mobile."Between Catalunya and Assen I felt good and then I had the same problem as Catalunya in Assen," he said. "But from Assen to here I haven't fully recovered and I definitely don't feel 100% going into this weekend. I'm doing as much as I can and doing all the preparation I hope will help and if I can perform a little bit better than in the last two races then we should be a little bit more competitive. I've just got to try and last the weekend."In Catalunya I was having cramps but in Assen it was just vomit. It was two different kind of symptoms, but I was left with no energy and we're trying to understand why that is. I'll be taking a few things this weekend and if it helps then I'll know we're on the right track and maybe to have an understanding of why it is happening."The course of action in Monterey is to take medication "to try and get my energy levels to last a bit longer and also some things for my stomach just to settle that down and keep it calm. It's not the best time to be sick when there are back-to-back races but we just have to see how we go."