VCV Test Results- Surge accompanies slipping

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I own a 1981 300D. It has slipping issues. To test my transmission vacuum control valve, and following the procedures outlined in many threads found here, I ran vacuum line from the vacuum control valve (VCV) through to the cabin and to a test gauge.

As I drove I noticed a few odd things that both troubled and reassured me. I did this after driving with no vacuum to the transmission as a test, and had no slips (but late shifts and no 4th gear at 70 mph). What I noticed was that I have 14" vacuum at high idle, dropping to 0" at full throttle. I also have 0" at regular idle, curiously, jumping to 14" upon application of a little throttle or adjusting the idle.

As I approach 2-3 or 3-4 shift point, as usual, the dreaded slip occurs, but as I notice the gauge, a surge of vacuum follows, almost immediately. So, since the transmission is not a consumer of vacuum, I assume it is not causing the surge, but surely the surge at the gauge is also surging at the transmission, causing the slip.

I had a look at the VCV and saw that it was within specification. How easy is it to blame the EGR circuit or VCV/proportioning valve? (I will consider an EGR bypass next, but regret considering replacing a VCV)
Thanks, all!