Boost spikes are caused mainly by the wastegate not being able to control the boost properly. In some cases it is the boost controller and connections. Get a quality wastegate (Tial) that is properly sized for your setup..and check for boost leaks.

You can check for boost leaks by getting a 3" or 2" (whatever turbo inlet size you have) rubber hose at homedepot, two hose clamps, and a pvc pipe cap (matching the size rubber hose) drill a 1/2" hole in the cap and get a quick connect end for an air compressor line and screw it into the hole you drilled on the cap. Connect the rubber hose with the cap all secured by hose clamps to the turbo inlet. Next, connect an air compressor to your pvc pipe cap and pressurize it at about 10-15lbs using the regulator to regulate air pressure. Listen for leaks..

-Alex