Unfortunately this can be a problem with scan converters. Make sure that you've got the cleanest signal you can get going into them, so you can eliminate the server from the chain. Sometimes having ferrites on the VGA cable can help with this kind of thing.

I've not used the Corioscans personally, but I've seen this kind of thing with Extron VSC700D's - specifically with 75Hz.

The ulitmate scan converter IMHO is the Barco/Folsom Image Pro HD, which gives a very clean signal.

Hugh