If the TB isn't dirty and its still doing that it may be because you dont' have quite enough slack in the throttle cable. I had this problem on a brand new AS&M 58mm.

Adding an extra spring as someone suggested can work, but can also be tricky for find the right one at the right length.

My suggestion would be to take an adjustable wrench and with the throttle cable removed rework the black stamped steel bracket where the cable housing stops. Clamp the adjustable on tight and then torque on the bracket in different spots to move it a couple millimeters forward closer to the TB. It won't take much bending to get it right, plus you won't have a big ugly spring hanging off of there forever.

Good luck,
Brent

 

had a similar problem with my 58 mm throttle body but it was very sensitive when i gave it gas so my mechanic just gave the return spring another turn and tightened it up

 

I had this problem and I soldered it, now always return to the same point.

 

it looks like it was soldered, but broke, maybe I'll put a dab of JB Weld on it! lol