Run the combined H+V sync wire to both pins 5 and 6 on the monitor.
E.g.
13 + 14 on the VGA card, combine into one wire (as you already have)
then
connect the one wire to both pins 5 and 6 on the monitor.
If this doesn't work, then separate them and run one wire from pin 13 to pin 5 (mon) and 14 to pin 6 (mon) or vise-versa.
Also, you might want to connect the other ground connectors on the video card to pin 4 on the monitor. There might be a difference in ground between RGB signal ground and Sync ground. Connect them all together (pin 6,7,8, plus pin 5, 10 (also grounds) AND the shield ground from the connector itself (wire braides inside cable) into pin 4 of the monitor. Do this only if the fix above doesn't help.