Expression based wrist->forearm rotation unstable?

Nimzo

So I recently made a rig where the forearm's rotation is driven by the rotation of the wrist. I used the expression editor and used an expression like:
L_forearm.rotateX = L_wrist.rotateX / 2; 

It seems to have work to some extent, as it produces seemingly correct results 80% of the time. To note, I use an IK solver is placed at the forearm but the effector was moved to the wrist. 

The most noticeable problem is illustrated above with pictures, the arm knots up. It also twitches a lot when moving the wrist control.

Are there better approaches to having the forearm move with the wrist in an IK set up? or is there anything obviously wrong with my setup?

Thanks