Thanks to all - here's a tutorial.

Blitz3D Forums/Blitz3D Programming/Thanks to all - here's a tutorial.

Yeah, the reason for the 1000 spheres (and windowed) was so that it would deliberately thrash the graphics card, forcing the need for tweening. Then I could mess with the code and really start to understand what was happening.
Hope it's helped to clarify tweening.
BTW I've heard that some people get a regular glitch using tweening, but I've never had it.