I think, first of all, double check your wiring.
You should be using 6 points from the PS2 A/V output.
Pin 5 or 6, 8,9,10,11,12 based on this table
R, G, B (pin 9,11,12) wired directly from the PS2 to the VGA input of your LCD.
Then for the LMH1251 board, you need to have 3 wires on the input...