After a considerable amount of debugging, (with the help of @agp.cooper and @Alastair Hewitt ) I was able to get solid 40x30 text output from my 8-bit video board! I'm using a 8x16 font with the resolution of 640x480@60Hz and driving it with a pixel clock of 12.5875MHz, which is within the range of the clock speeds supported for the MC6845. I've got a ton of stuff to test before making a new revision of the board, but..... it works!