3rd Gear Problems - Heading to dealer, any advice?

Last time I was at the track last year, I thought I missed a couple of shifts into 3rd gear - always on the same corner. Just seemed like when I used normal motion/force, it just never got into gear, and I had the clutch out before I realized it. No grinding, just had to try again, and push a little harder, and in it went. It's a long, sweeping left hander (flat) and I make the 2-3 upshift at about 6k rpm. Assumed it was just my own stupidity, didn't think much more about it. Then, yesterday, back at the track, same turn, same shift, but this time it actually grinded (ground?) going into 3rd - and I KNOW I didn't muff the shifts this time - something's not right.

I don't seem to have any trouble under normal driving conditions, but shifting in high-g situations is not good. So, I'm considering taking my car to the dealer as my warranty is up in March.

Few questions:
1) should I put the stock shifter back in before I take it in
2) is this something others have experienced (I see lots of 4th gear probs, but none about 3rd)
3) how much should I tell/not tell the dealer about what I do with the car

Car is all stock except for alignment, pads, and BPP shifter. 17k miles, never drag raced (at least since I got it at 10k miles), autocrossed often, track days whenever I can.

Thanks for any help!