Make it shift earlier and harder. The control cable does the when and the modulator does the how hard.
I would try one turn on the cable housing nut. (white 14mm plastic nut at the top of the cable housing and one turn in on the modulator on the front left side of trans (the left is always the drivers side [at least in the non British world]).
The concept on the cable adjustment is to get the shift before you start letting up on the throttle. For a little more finess read this article: http://www.continentalimports.com/ser_ic20242.html