I just buy a Cervelo P3C with the new 3T fork. I used it once on the road and few times on the wind trainer without problem. But yesterday I noticed that the fork is not stable enough in the head tube when I used the front brake. The fork is playing in the frame. I tried to fix it but couldn't succeed. Any help or advice highly appreciated.

Your headset is loose.

Loosen the bolts in the stem where it clamps onto the steerer. Then tighten the bolt in the top cap of the fork until the play is gone, this is known as pre-loading the headset. ( The best way to do this is to hold the front brake and try and move the front of the bike back and forth. ) When you are happy that it is just tight enough so that there is no more movement tighten up the stem bolts ( to the recommended torque ) and you will have not more problems.

If you're still a bit unsure, take it to your bike shop and they will sort you out.

More info here:

See section: Headset Adjustment - Threadless Type

 

http://www.parktool.com/repair/readhowto.asp?id=65

 

Allen.

One possible problem I've seen before may be the height of stem+spacers is too close to the top of the steerer tube so it contacts the compression cap... might need a thin spacer above the stem (or replace current w/ one slightly thicker) so there's a slight gap between the top of the steerer and the compression cap to give it room to squeeze downward.

 This exact thing happened to me on my new RS.  I noticed a loose headset after my first (25mile) ride,  went into the shop and owner adjusted it.  Went for second ride next day,  handling became at at 35 miles,  noticed  loose headset,  cut ride short and  brought it back in.
Mechanic put a small spacer on top of  stem so the cap could  properly clamp down on the bearings  and its been rock solid since.