I think there's a problem on the car somewhere. The fact that the boost drops to about half of what it was I'd say the amal valve is engaging and switching to low-boost. Either cause it's knackered or because there is a fault in the electrics or some other sensor. Or - as mentioned above, the spring in the actuator has gone weak.

Adjusting fueling is a piece of cake if you have a wideband lambda to check the result. You just take of the pressure actuator (black box on the metering unit). Make sure you don't lose the O-rings and remove the big normal screw on the back of it. Behind it is the adjustment screw (allen type). Clockwise makes it richer, anti-clockwise leaner. However it's very senstive and get's even more so, the further you turn it. A 1/4 of a turn already does quite a lot.