Casey Stoner remains unsure about the cause of the illness that has hampered him for the past two races - and says he cannot be confident that it will not hold him back again at Laguna Seca this weekend.
Although the Australian took third places behind the Yamahas at both Catalunya and Assen, in both races he had to ease his pace due to discomfort from stomach cramps.
"The situation is rather strange and difficult to understand," he told Gazzetta dello Sport.
"Between the Catalunya GP and the Dutch TT I felt fine, but then in the race I felt sick again. Now I've had little time to recover and in these days, to be honest, I've never really felt 100 per cent well.
"At Barcelona I had stomach cramps, while in Assen I threw up after the race - two different symptoms, but perhaps caused by the same problem."
He hopes he can find a way to improve his condition before the Laguna race.
"We'll do some medical tests and we'll try to understand the reasons," said Stoner. "The objective, obviously, is to get to Sunday in better shape compared to how I was in the last weeks."
Stoner added that the situation was extremely frustrating because he believes the Ducati is competitive enough to fight with the currently dominant Yamahas.
"The potential to do better was there, and that could be seen at the start of the race both in Barcelona and in Assen, when I could keep up with the Yamahas' pace," he said.
"Unfortunately after that I had to give up. You could say that in the last two GPs the limit was myself more than the Ducati.
"At Barcelona we made a good step forward and the GP9 works well. [Valentino] Rossi and [Jorge] Lorenzo are going rather quick, but if the bike carries on being this competitive then we can get some good results. However, a lot will depend on my physical condition."