Portamento is an inherently difficult thing to do on a sample playback device. This is because the unit has to bend one sample up (or down) and at some point grab the next sample from the next key range and have it bent down (or up) and then start bending it. Going from one sample to another smoothly without any noticeable artifacts is almost impossible. This is why portamento is not normally found on sample playback units. In fact, it may be different now, but when the K2000 was released, it was the only sample playback unit that could do portamento.

The only solution to the problem is to edit the keymap so that you have the a particular root assigned across the entire stretch of keys where you want to do your portamento. Then you will only be bending a single root instead of transitioning between two or more roots and you won't have the problem.