same problem i have, but i got the xenarc 700tsv. When the screen is on,(has power), I have to be close to the car to use the alarm. When the screen is off, alarm works fine.
My alarm is stock, came with the car.
What i did to disable power to the screen completely, (cause the screen has standby), I put a relay on it and the relay gets power only when the pc is on.
This worked for me.
and before anyone starts, i spent months troubleshooting each part to find out it was the screen.