I can't confirm this is a problem with any other setups, so here is what I'm working with: Pentium 4, 2.3GHz, 1GB RAM, Windows XP Home, Sonar 5PE, Delta 66, GPO VST
THE SHORT STORY
Selecting very high latency settings breaks GPOs sound fluidity for both legato and strings of short notes.
THE LONG STORY
A piece I had been working on for a long time suddenly starting sounding different in a very bad way. At first I noticed that a trumpet line that was full of staccato notes was randomly dropping every second or third note in the line. I slowed down the tempo, and they started appearing again. I "fixed" the problem by altering the lengths of the notes, but was not happy.
Then I noticed that a bassoon solo that contained grace note slurs to longer notes were no longer being played as legato. The only "fix" for this was to make the grace note longer, but it was no longer a grace note at that point! And even after that, the legato worked randomly about 50% of the time.
After trying many things, such as turning on/off DFD, reloading GPO instances, rebooting the PC, restoring an older version of the Sonar file from backup, I was tearing my hair out, and practically in tears.
Finally, I tried turning the latency down from what was a fairly high setting. I think it was set in the neighborhood of 300ms. Suddenly, everything was fixed.
Normally, I work with a fairly low latency (5-30ms), but it seems I had jacked it way up when the piece started to choke on the very last extra-busy tutti measure of the piece.
Lesson learned. Something about GPO, Sonar, or my sound drivers just doesn't like the GPO samples at high latency. From now on, I'll be freezing tracks if lower latency can't handle large sections.