If you are using twisted-pair wiring, never send DIRECTION and STEP down the same pair. Use one pair with STEP and COMMON, use another pair with DIRECTION and COMMON. Connect both pair's COMMON to the G203V COMMON terminal.
Run your 1.25A motor windings in parallel and set the G203V to 2.5A with a 27K CURRENT SET resistor value. You will get much better performance at 36VDC that way.
The G203V doesn't need any heatsinking at 2.5A so you can mount the drive any way you like.