importance of the speedo
i also know that the ecu needs road speed readings fed to it via the reed strip in the speedo cluster. i'm not too certain but there's something about the ecu metering power if the revs go past a certain threshold without there being any road speed indicated (by the speedo)..
so, if i were you, i'd check:
- the actuator arm/spring for freedom of movement & that it holds the WG shut
- check the actuator vac pipes
- remove the intake side plumbing and check that you can turn the turbine blades
- if running the stock actuator, remember that it's the bleed type actuator and vents some of the boost pressure via the exhaust nipple that goes up via the front side of the cam belt cover. if this is blocked, it will exponentially build pressure in the actuator thus keeping it open all the time
check these and report back..