I'd check the actuator, could be a little weak and unable to hold the extra boost pressure.

Also, check your boost hoses (air intake, turbo to i/c and i/c to throttle body), see if they are squishy. When the turbo draws air in, if the hose isn't strong enough it can collapse, leading to an insufficient air supply.

Fuelling (pump, pump electrical supply, filter, injectors) and plugs would be the next thing I'd check. As boost goes up, you need more fuel and a strong spark

Hope this helps.