Hmmmm, your HorizSync 31.5-37.9 of monitor does not make to much sence to me. There is indeed no mode left to be used.
Use the program gtf and try it yourself:
gtf 1024 768 37.5 (resolution x y and refresh)
The answer you get is
# 1024x768 @ 37.50 Hz (GTF) hsync: 29.44 kHz; pclk: 37.68 MHz
At 37.5Hz, hsync is 29.44kHz...... not within the ranges you defined! Nothing will fit, sorry.
Are you really sure about the values?