There are a couple of things you can do to reduce the flickering, but not eliminate it completely until you get another monitor.
First, you could use NTSC instead of PAL, which still flickers, but flickers faster! It makes the flickering slightly less noticable.
The best solution would be to switch your screenmode to 16 colours and use the MagicTV patch from Aminet. ClassicWB won't look too good in 16 colours as it is, so you'll have to edit and lock the palette. Since you're using ClassicWB ADVSP, I guess you're using Scalos, and in this case you need to use a special locked palette for it. I'll go and lock the palette and come back and post it.
Anyway, here's MagicTV -
http://aminet.net/package/util/wb/magictv2