I think this will be the best solution Nebula is cpu/ram intensive, so you need a proper hardware. The difference could be huge: using a good pc you could open "dozens" or even "hundreds" of instances (someone reported more than 100 eq using a fast pc and windows x64) instead of few.
I've got the same problem. When I open a new Projekt in Cubase 5, open a Synth and load one instance of Nebula and load a preset,everything is ok.
But if I load another instance, somethimes the audio disappears (somethimes only on one channel). I can save the Cubase Project and reload it again. Then it works. I've 4 GB Ram, Core 2 Duo Cpu,Windows 7, HDSP9632 and Buffer is set to 2048.