Sync/dropout problems using plugins in Cubase SX

I'm using Native Instruments Pro 53 and Battery - two instances of each.
When my machine hits the point where all 4 are playing at once, I start
getting audio dropouts on the Pro-53 tracks (the rest of my audio tracks
including the two instances of Battery play fine). It doesn't do this every
time - only after I've shuttled the locator back and forth a few times. The
only way to clear the problem is by shutting down Cubase SX.

What's odd is that I'm running a 2.4Ghz Pentium 4 with a 533Mhz FSB, a
10KRPM drive with a 1gigabyte of RAM with no other apps running. I've
tweaked a bit with the ASIO buffer count and that hasn't helped at all. And,
the CPU utilization in both the Windows task manager and Cubase's "VST
Performance" monitor have the CPU at 15-20% and the disk at 0%!

Any ideas what's going on, here? Should I detatch it and let it run in the
background? Anyone else experienced these problems? Thanks in advance!

-->Neil