I'm a long time triathlete, but I'm just now on my second bike. I spent about 18 years on a Cervelo P2k. It was a great bike. It handled like an aircraft carrier, and that never bothered me. I didn't notice crosswinds at all. I recently bought a P3X and I'm finding that it is very sensitive. The P3X is like a formula 1 car, strong cross winds have me nervous and getting out of the aerobars frequently.

I'm hoping you guys with more bike experience can help me figure out why it is so twitchy. I'm wondering if I got the wrong frame size. I bought a medium frame, but I would fit in a large frame as well. Is this the reason for the bike being overly-sensitive, or is there a frame geometry change that explains it?

Here are my numbers from a well respected fitter:
Arm Pad Reach (mm): 439
Arm Pad Stack (mm): 663
Saddle Ht (mm): 804