Resolution problems


I have some problems with OSX at the highest resolution allowed by my monitor (a Formac 17/500) when Classic is running and there are millions of colours. The bottom screen becomes fuzzy, with vertical lines and the cursor leaving a sort of strain behind. If the colours are set to thousands the problem disappears.
My Beige G3 has the VRAM memory at its top and I don't have any problem with 9.1 or 9.04 runnung with full resolution. Any idea?