I'd say it was the boost sensor vacuum reading that is causing the issue. Remove it from it's current position, and T it into the plenum vac take off, or the FPR vac take off.
On my AVCR, it advises to T into the FPR vac line, for the boost sensor, with as short a pipe as possible, this is pretty close to the plenum.
Logically, as close to the plenum as possible, before the throttle butterflies, is the best place iirc, then you're getting the exact pressure reading that the SBC will be interested in.
Also, it might be worth making sure that the hoses between the solenoid and the actuator and the plenum and the solenoid, are the same length.
Also, using little cable ties as pipe clamps, if not done otherwise, secure all the vacuum pipe connections...... You never know !