The NG FMS is a bit of a disaster area in the descent. I normally prefer manual control of the autopilot and use VS quite often to control the descent better. In your case, to do it cleanly and prevent those weird speed reductions, input a new cruise alt at FL300, ensure the speed is what you want or reset it, re-engage VNAV, and wait for your new TOD to hit YYY at FL280. Untidy, but leaving VNAV sometimes produces some bizarre speed reductions that I haven't worked out yet .
The 737 VNAV descent program is a disaster compared to the 747 one which is far better.