ML Wi-fi Problem

I've done a lot of Googling and searching here but I still can't sort this very annoying problem since upgrading to ML:
I have a 27 Inch iMac Late 2009 Model
I have slow internet speed, so in order to download files I have to leave them downloading overnight.
However, when my Mac goes into sleep mode the wi-fi is dropping. When I wake it up from sleep, there is no connection and obviously my files haven't downloaded.

I've reset SMC, PRAM, and also set up a new location all from tips around the net but still can't get this sorted.

I've been a convert since Tiger, and this is the first problem I've ever really had but its very frustrating