As the name implies the notes on the uper half of the keyboard are multisamples of a staccato performance.
They are not looped and will only sound as long as they have been sampled. You cannot change that with the sustainpedal or envelope.
If you have to repeat long notes i would suggest you use different velocitylayers for the repeats. That way you can avoid the machinegun effect at slow tempos.
If sustain is actually working, I'd say to do a sus on with the ote ons and sus off at each release, to avoid the mud. If you just hold it down, it will indeed get gooey. Also be sure your envelope decay times are long enough, though it's probably a sample length issue.
------- It's all about the music - really. I keep telling myself that...
simply by playing at different velocities or edit them in your sequenzer.
The trick is to use neighbouring zones on repeating notes. That ensures that you will have a different attack on repeats but dont get too much of a volumechange due to the changing velocity.