How's your connection if it's wired directly to the Modem?
Is the issue both wired and wireless on the Billion router?
Have you updated the firmware of your new router? You might want to check the manufacturer's site and see if there's a new firmware available then manually install it.
Please post a Xirrus snapshot coming from one of your wireless computers. See this instruction from the Networking Sticky.
Also, please try not to overlook any of the suggestions that I have given you, it is important that you follow our advice.