For the second race in succession Casey Stoner was forced to battle physical problems on his way to a podium finish, while Fiat Yamaha title rivals Valentino Rossi and Jorge Lorenzo disappeared out front.

As in Catalunya, the Ducati Marlboro star needed medical treatment immediately after the Dutch TT at Assen - and this time he couldn't even attend the podium press conference.

Worryingly, the exact cause of Stoner's energy-sapping condition doesn't seem to be known.

At Barcelona, the 2007 world champion had suffered stomach cramps and a sleepless night, stating vaguely: "I wasn't ill but I wasn't quite myself".

No more was said publically about the problem and he was assumed to be fit and healthy this weekend, and began the race from a close fourth on the grid.

However, after leading the field over the line at the end of lap one, Stoner was passed by Rossi on lap two and Lorenzo on lap five. While he remained unchallenged in third, Stoner finished a massive 23 seconds behind Rossi.

Ducati stated that 'Stoner suffered throughout the weekend with the physical condition which had already affected him in Catalunya worsening as the days went on' - but Ducati MotoGP director Livio Suppo admitted that the cause of the problem is still unknown.

"There is something wrong with [Casey's physical] condition. We are trying to understand and help. What we have done so far is clearly not enough so we have to investigate more," Suppo told the official MotoGP website.

"The Clinica Mobile has been very useful after the race. We have spoken with them and they have some ideas for Laguna. So let's hope.

"At the moment it is difficult to say [what the problem is] so let's trust the Clinica Mobile and hopefully they will help Casey," he added.

Stoner revealed that 'nothing seems to have worked' in terms of treatment and regaining fitness in time for next weekend's US GP is now his number one priority.

"I've been feeling fine all week but started to feel run down again on Friday and I've just got worse as the weekend has gone on," said Stoner. "I've taken every kind of supplement and vitamin tablet you can think of but nothing seems to have worked, so it is obviously something we need to have looked at closer because I can't keep going like this and it is costing us points.

"Thankfully I had a fantastic bike underneath me again today and I owe the team for that. A big thank also to Dott. Macchiagodena and our physiotherapist Freddie [Dente], who took good care of me both in Catalunya and here. Unfortunately we don't have much time to get fit for Laguna but we'll see what we can do," he added.

Stoner, who began the Dutch TT equal on points with Rossi and Lorenzo, is now nine points from the world championship lead.