You're spiking, not overboosting. The ECU is overshooting the initial target on approach and settles down eventually. It's not great but not the end of the world either. BTW, if you're reading your boost off the gauge, it is likely slightly higher than what the ECU is seeing in the manifold which means that your actual boost error is not as big as you think. Your gauge is likely reading off the IC which is subject to some pressure loss before the MAP sensor. The only way to accurately assess the situation is to log the car and see what is reported. Your gauge could also be inaccurate and you're sweating for nothing.