Not familiar with the -45's, but on the 67's you can swap the prop sycrophaser cannon plug on the Prop Gov. with the auto-feather dump solenoid plug. Prop-synch signals will begin to feather prop! Double check.
Do a ground run with prop-sync on/off.
This very much also sounds like a ground idle solenoid issue.
Once again, never worked on 45s, but does it have a ground idle solenoid? It looks like it does (you have a Ground Fine lamp - or the dreaded old -20 Secondary Low Pitch Stop). Check the wiring/operation of the ground idle solenoid. It is located on the forward end of the propeller reversing cable linked to the beta arm.
From the 1900D manual (-67Ds):
You're ground idle solenoid will be connected to something like a squat-switch/safety switch on the landing gear. Will provide power to the solenoid to bring the beta-arm back. Check rigging/wiring of that switch. Wish I had some Shorts references for ya. It sounds like the solenoid isn't releasing as it should when in the air, then intermittently does every-once a while, which equals greater prop pitch, resulting in the torque increase and RPM decrease. The fact you are not seeing any change in N1, ITT, or FF when this happens leads me to believe this is a propeller issue, not a engine/bleed-off-valve issue (you would see N1, FF, and ITT changes with a bleed issue).
Anyways, i'd swap the ground idle solenoid, check operation, check squat-switch rigging, and rig the forward end of the engine along with adjusting propeller ground and fine idle torques. Along with checking syncrophaser operation (if installed).