Midi/Rendering problem

[Rock Hardbuns]

Ive been having these problems lately.
I record a pad with a VSTi. I shrink the clip down to the part I want. Then I quantize to 1 beat and copy paste the clip a couple of times. I may also do a "delete content outside of the clip" thing.
When I play it back it sound all right. BUT when I render it, I get hung notes-o-rama.

I think what happends is I get notes that are to small to show up in the editor that dont have note-offs, or maybe the note-offs of the notes that are supposed to be there get deleted?
I dunno. I have gotten around it by limiting the polyfony of the VSTi so far but Im wondering if its happening to anyone else? Is there a workaround or something.
And why do the notes only hand in the rendering, not in the playback?