could be several reasons why its over boosting,
first things to check would be all the vacume/pressure lines between the waste gate actuator and turbo compressor output line are connected to the boost solenoid and are not leaking, check electrical plug into boost soleniod is not damaged.

if thats all ok use a small shifter and manually open the waste gate to make sure its not seized or sticky.

if its still causing problems put a mechanical boost controller in, and bipass the boost solenoid.