[Liko]

Well, it may actually be a timing problem. My car recently had something similar after the water pump was replaced (meaning they had to strip all the belts). The car hesitated when you tried to push it beyond 3500 revs. It was solved by retiming it, but now when it's cold the hesitation's there until the engine warms up. You may have yours timed on just the opposite side of perfect.

Alternately, it may be a mixture problem, or something with the throttle. I wouldn't know as much about that, but I know I had to turn my car's idle revs up with the cold weather.