Herring... and Charles,
Yes, now your situation is beginning to make more sense... the 2mm larger orifice was originally necessary for it was supplying vacuum to both the Tranny-shifter system as well as the EGR system so more was needed then. But then I believe you said the EGR was disconnected... so this left too much vacuum and the double switchover valves were played with so the VCV was/is probably hooked up such that the VCV bleeds down the vacuum even at idle when normally the switchover valve would NOT be open to the VCV and allow it to bleed off the vacuum.

Assuming I am correct that you are no longer using the EGR, I would try to figure what ports connect to what other ports at idle and at higher throttle positions. What I'm getting at, is that I think you should try to restore the switchover valve or valves necessary to restore the switching ON/OFF of the bleeding action of the VCV. There are subtle reasons for this action that is based upon throttle position... e.g. possibly to prevent the car from down shifting too soon... or too late! So using the "puff/blow" method I described earlier, try to figure out basically which of the 5 ports connects are interconnected at idle and higher positions of throttle. I think you will find that #1 & #3 connect and that #2, #4, & #5 interconnect. You need to first confirm my speculation... and then work on restoring those that control the shifting [ #2,#4, & #5 I think]... making them vacuum tight. These switchover valves are not too expensive [ ~$20 to $30]. Then IF the vacuum level being sent to the tranny vacuum modulator is too high or low, then we can get into how best to adjust this... including adjusting the VCV!

Keep in mind that on our 1980 models, we have a simpler switchover configuration... especially when you take the EGR system out of the picture... these cars only have the one valve that simply turns ON/OFF the VCR bleeding action. Take a look at: http://www.peterschmidtransmission.com/vacuum/1980/617_912.jpg
We are working toward such a simpler system for you too IF I'm correct that you too have abandoned your EGR!??
Regards,