Thats because youve added too much preload, so that your wastegate cannot pass enough volume of air at high RPM, thus the turbine speed begins to increase.

That is EXACTLY why you i said you dont want a -31 for 18psi with no additional boost controller as 14psi requires around 18psi base load and a -31 is no good at all for that. Also, how do you plan to use a Dawes device to make your boost LESS aggressive? Its whole point is to make the boost come in harder with no gate creep.. same as fitting a -34 as its simply a pressure relief valve.