My experience is the same as yours. Asus made a horrible decision to use no heatsink on the southbridge. When I first got mine, I was using a 4600+ with PC6400 and a slight overclock. The system crashed a lot. I knew I had good RAM, so I looked around inside the case to see what the problem was. The heatsink on the northbridge was hot enough to the touch that I couldn't keep a finger on it for more than a few seconds. The southbridge was so hot that couldn't believe that it wasn't melting. I couldn't even hold my finger on it for a second.
I installed a heatsink, and it stopped crashing. If you look at every other board on the market that uses this chipset, they all have small heatsinks on the southbridge, so it doesn't really make any sense why Asus omitted it. I like the board (it's the only one I could find with on-board DVI), but adding a heatsink is a must.