Just a hunch...

I've got a 486 with a Phoenix BIOS which did some shadowing at 16 MB. When I increased the RAM to 64 MB I had to turn that off. What had been a nice sharp "beep" on bootup became a dull "boooooop". The BIOS messages on the VGA text screen became much slower too. Once it flipped into protected mode on booting OS/2, it became a non-issue (since OS/2 in protected mode doesn't use the BIOS).

Maybe there's something similar going on with your system, but it's likely that it just doesn't do VGA very well.

Cheers,
Scott.