VSTi problems in Cubasis Options

[Craig]

I've been having problems using VST instruments in Cubasis on my iMac.
Though I can 'play' the instruments okay, and input notes via the Score Editor,etc. when i come to play them back after recording, notes seem to be dropped at random. It seems to work a little better at lower tempos, but there are still problems.
Stranger still, it seems to have even greater problems with the Virtual Memory switched off. I've tried increasing the memory assigned to the program to well beyond 200MB, but it made no difference.
I would greatly appreciate any suggestions anyone has to offer.

Cheers

Craig.