at 3800kV i'd say a 9T or 10T for 3D, and an 8T for sport or training.

i'm not 100% sure, but i think your problem could be caused by the following: 1) you're not used to the extra pitch sensitivity so you're moving the stick more than you need to to maintain altitude, 2) you have excessive slop in the mechanics, or 3) your rx or antenna are too close to high current.

1 & 3 are easy to understand & fix. if it's #1, usually some expo fixes the problem. if you don't have expo, just try taking it slower and feeling it out before you slam on the gas. if it's #3, i've seen at least 5 clear cut cases where glitches resulted from the installation of the rx and/or antennae within 1/2" of high current devices & wires. take care to keep the rx & antennae at least 1/2" clear of any high current (escs, battery, battery wires, & motor wires). #2 is more complicated; read below for further understanding.

higher headspeeds can in fact cause problems with mechanical slop, because higher headspeeds give more of what i call "pitch sensitivity". to explain what i mean by pitch sensitivity; consider the blades are traveling faster at higher headspeeds, so it takes less angle of attack to create the same lift. alternatively with the same angle of attack, higher headspeeds create more lift. now say you have 5deg of slop in the head, from say a loose collar for example. with normal headspeeds, when the blades jumped from 0 to 5deg, the resulting jump/drop in lift was easily manageable. with higher headspeeds, when the blades jump between 0 and 5 deg you get much larger jumps/drops in lift, resulting in a difficult to handle heli. basically, when you run higher headspeeds it becomes more critical to get everything perfect & slop free.

i won't go in to detail how to remove slop unless you know slop is the problem (it's hard to describe my process of hunting for slop, would take another long post). to measure slop you can check your slop by folding a blade back & holding it still, then move the flybar. the angle that the flybar moves is your total slop for that blade. a healthy blade cp pro should have no more than 2-3deg slop. i've seen 10deg of slop fly on stock cp2's, but it's no fun and definitely isn't what you want for a brushless pro.

cheers
-kev

[edit: i forgot to mention, you should have a looser gearmesh. the rotor should spin freely, only being slowed by the cogging of the motor's magnets. looser is always better than tight; that's what i always tell newbie pinion setters. also, i forgot to mention the bell hiller arms. make sure they're not too tight or loose. tighten them up, then back the screw out just until there's no binding, but not so far that they move around a lot.]