Sounds to me like your upsetting the suspension of the car right at a critical moment by catching that last gear in the middle of a corner. The object of good cornering is to make as few changes to the weight transfer, line, brakes, and, gas as possible. I trail brake quite a bit in my Pro5 when entering corners to help the back end rotate more which is just a smoother way of transfering grip and weight than what you are doing and still getting the desired effect. If you want the car to rotate less try and make the entry and mid corner actions a little smoother. If you absolutely have to make that 2nd gear shift mid-corner like you are describing, try and do a little heel/toe shifting to smooth the transition out. I use a lot of engine braking like you describe, but I heel/toe my downshifts to keep it from upsetting the suspension of the car. If the entire corner is not one smooth motion you are going to end up with things sliding.