Difficult Guinea Pig

I have just bought a new male Guinea Pig, Tincker.
He is really friendly, eats from my hand and lets me stroke him! I have only had him 2 weeks and already I have learnt him to do the circle trick with his favourite treat.
However, just in the last few days he has been behaving strangly.
He wont let me pick him up with out a struggle, once on me he is ok.
But he keeps on having these wierd moments, where he will shake his whole body uncontrolably, like shaking his head really fast and banging about the cage, like he is being attacked.
I did not touch him and he only does this at night.
I thought it might be fleas as he was scrating too.
I checked his whole body and nothing. No fleas, no injuries. I have still applied the flea powder though and gave him some more chewing sticks.
I cant explain it!
Whats up with him? Is it fleas?
And if he is being difficult- how can I stop him from doing so?

Thanks all!
Hamsterhaven.!

Take him to the vet to check for mites. That's how they act when they have mites, and you can't see them. Flea treatment won't help with mites. He needs Ivermectin, usually given by shot by the vet, but I think it's also available in oral or topical form. The vet will have the right stuff. Get him there ASAP, as he's very miserable and will only get worse.

Good luck!!!

Ok. Thanks for your help.