BM, you pointed out some good things on here, which may or may not be part of the problem, especially tear and wear on the drivetrain. The pins and cups are wearing out real fast on the MGT7, I usually change them at least every two races. If the cups are marked, the suspension stops working as it should, your car will start kicking and the rear end definitly is going to be unsettled and might even brake loose.
About the diff oils, comment from my side. i have found the car to be less consistent with heavier oils in the rear diff mostly because when you loose it, it's gone. For example with 1 million to 500k, the rear breaks out, there is just no way you get it back under control, it just slides, slides and spins...
Comment about tires, we use here in Brazil mostly either Alpha or Taher tires. They both are made in Brazil and are pretty good, and Taher will be the offical tire for the World Cup in Argentina. If you can get your hands on some of them, you might be surprised. As far as I know, the Alphas are as well sold as Flashpoint tires through Mugen...