Hey dude, I had the EXACT same problem! Car would do fine until the clutch/transmission got hot (from a couple of runs) then it would constantly slip under high RPM's or when shifting while racing. I hope your causes are the same because it'll be a simple fix.
I thought my nismo pressure plate was the cause for mine so I swapped it for an OS Giken twin plate and it held great for quite some time then started to do the exact same thing! So, I was starting to check the clutch out when I pulled off the slave cylinder and noticed that it extended out on it's own rather quickly. I tried to push it in an I could not do it by hand, I had to force it in using the transmission housing. I figured the master cylinder was shot and already had a repair kit so I rebuilt the master. It was the first one I ever did and checked out how it worked. I noticed a pressure release valve that is hit when the pedal is all the way back and it only has about an 1/8 inch of movement if that.
After putting everything back together, I checked the clutch pedal and undid the switch stop that dictates where the pedal stops when it is out. As I screwed the switch back, sure enough the pedal was also coming out. I did this untill the pedal stopped and retightened the switch in place. I double checked the slave cylinder and sure enough, I could push it in by hand with little resistance.
I don't know if this is the same cause of your problems, but I hope it is just for the ease of repair!