Both the initial AirBoot screen and the subsequent menu run at 640x480. This is no different than the OS screens that follow it, and on my system, the BIOS screens that precede it. It seems unlikely that the monitor doesn't support standard VGA.
I'd suggest reinstalling AirBoot. However, don't use "mini-LVM" (AOS's simplified LVM utility); instead, do it from the command line. The reason is that mini-LVM only replaces AB's code when reinstalling. In the few instances where I've had problems with AB, the cause was corrupted configuration data that needed to be replaced.
On AOS, you'll find the installer ('airboot2.exe') in x:\sys\bin. The program has 2 commandline switches: "/forcecode" and "/forceconfig". You may want to start with just "/forcecode" (identical to what mini-LVM would do). If that doesn't fix it, try adding "/forceconfig". The latter is more likely to fix the problem, but it will require you to reconfigure AB's menu because all of its existing partition info will be wiped out.