its possible that you havnt connected the vacume houses back up properly.
this may cause the turbo to over spin.
the standard chip wont like any thing over 10psi.
it is also possible that the actuator is not the correct type for your car.
as a last bit of advice, you will only really get an accurate reading foot to the floor in 3rd or 4th, the car also has to be at normal operating tempreture.
if this still doesnt work then i would saggest checking the amal valve (little brown device with vacume hoeses going into it, i think its near the master cylinder on fiestas). if this is faulty it will cause your to become 'confused' (for want of a better word) under boost.
hope this helps mate

lee