flat bottom blades are generally more aerodynamically efficient than symmetricals, so they take less power. you usually need to reduce the pitch of the blade grips when going from symmetrical to flat bottomed, since flatties have a built in positive angle of attack.

sound like you have too much pitch, which reduces rpm and boggs down your motor. in this situation your motor will draw too much current and you will not achieve the maximum lift available from your heli. adjust the pitch links (the ones above the swash, not the ones attached to the servos) so the heli just gets light on the skids around 1/2 throttle. this usually means you'll end up hovering around 3/4 throttle like DVI said.

the other possibility is your main motor is going bad. if it is, you'll have similar symptoms.