I've experienced that problem at times myself. The Portamento implementation in both GPO and JABB seems to be squirrely, and so I rarely use it.
BUT when I've had problems of notes sounding out of tune after trying to use the function, invariably I've found that I hadn't given the Port a full off "0" value at the end of it. Make sure that isn't the source of your issue also.
Thanks Randy! I'll check it out? I think in some of the Jazz/Big Band Demos posted in this forum may have had the same problem which was corrected by "tuning" the notes with the tuning controllers or parameters.