If you're sure you're getting a proper vacuum signal to the modulator, the modulator isn't leaking (e.g., drawing ATF into the VSV) and the bowden cable is adjusted correctly, I suggest you look into a Superior shift kit -

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=260116350141

There are a couple of long threads from the October 2008 timeframe that discuss installation and results. Spring tuning won't overcome vacuum problems and makes the transmission very sensitive to bowden cable setting.

edit - Make sure there's a cap over the modulator adjustment handle.

Sixto
87 300D