Sorry that I don't have solution, just a me too.
FreeBSD 5.3, vesa-patch for 1280x1024
But I have also choppy (slow) sounds, when I produce much output on a console (for example "find /")
This happens in all VESA modes on console, if I switch console to a text mode everything works fine.
But scaled console displays suck on a TFT ...
On different other sites I found hints that making a new kernel with PREEMPT and FULL_PREEMPT should help, but my attempts where not successful.