My '82 300D has a harsh 4th gear shift for the first one or two shifts of the day then its smooth. You need to get back to the basics.
1. set vcv to 10 -12 " make sure vac drops to 0" at wide open throttle
2. modulator holding vac (done)
3.adjust bowden cable so shifting is about 2500 to 3000 rpms
4. adjust modulator valve out to soften shifts
5.(Should be first ) Change trans fluid and filter add 1 pint Lucas Trans Fix and fill with mercon fluid to proper level
This is the procedure that I followed and the shifts improved with every adjustment. The biggest factor for making it shift like butter is the fluid, filter and Lucas Trans Fix change. Hope this helps . I live in Orchard Park NY, someday we'll get together for a beer.

__________________
'83 240D 264k on 100% biodiesel/ 31mpg
83 300D 217K the $800 gem
'82 300D 267K the new WVO project car finished and driving for free
International tractor B275 on B100
Ford 555 backhoe on B100
Sargent 25 ton Rough Terrain crane on B50