x86 emulator VGA fault?

[superfury]

I've written a VGA for my emulator. However, when I start it in mode 0x7 (80x25 monochrome text mode) it runs fine, yet every subsequent settings changing the video mode (including the current mode being reset on reboot), causes the left half of the screen to shift a bit and the right part to give junk.

http://superfury.heliohost.org/cplusplus/x86emu_vgacorrupted1.png

Anyone knows what's able to cause this? (I've based my VGA emulation on the freeVGA documentation. The script setting the video mode is from DOSBox's int10_videomodes.cpp)