Changing Clutch... Check Trans??

Well your tranny should be gone through at that mileage first of all. But if it pops out of gear then usually that gears is "blown" or there is internal damage.

I have to say though going to the track with it like that seriously isn't a smart deal, you only accelerate the condition to the point where you will eventually not even be able to use 2nd anymore by racing with it Expect a rebuild or replacement soon or just be prepared to start skipping 2nd gear.

On a side note, I am surprised it is second gear doing this. Usually it is 3rd that blows due to the poor design of the T5. How hard do you or have you slammed your car into 2nd gear if you don't mind me asking?

Why the hell are you putting in a used clutch dude? So not worth it. Would you put used brakes on your car?