You probably need to set this before starting virtuoso:
setenv CDS_WAVESCAN_OPENGL "-Dsun.java2d.opengl=False"
See solution 11610986 for more details. Essentially this works around a problem with the version of Java used in the waveform tool in IC614 (it's a Java problem, not strictly a Cadence problem). It certainly looks like the symptoms I've seen before. With VNC, the Java is being handled completely on the remote end.
In reply to Andrew Beckett:
Yes, that's fixed it. Many thanks.