I went through this last year, two things are causing this, weight balance and bias. Because the motor ass and speed are on the right, the left wheel unloads under power and breaking, this thread explains it
http://www.oople.com/forums/showthre...perfect+cougar
Now if you still see the rear unstable, you need more rear weight bias. Shave the rear arms at the inner and outer hinges to bring the hubs forward. Put the shocks behind the towers. The new inline steering arms will also help alot. Last two things, put the plastic shim along with 2 mm washers under the camber link. Then add weight. Trishbits sells a brass bar that screws to the chassis between the motor and battery. What I like is sticking lead golf tape under the chassis at the tranny. Hope this helps you.