I had exactly your problem till yesterday. Would either crunch or get locked out at WOT at 5000rpm plus.
I have changed to a cpe motor mount which made shifts alot nicer but did not fix the problem. Even tried the ssp but this made it even worse( yes i adjusted the shifter cable, for hours).
It all helped gear changes but didn't not help my 3rd gear lockout/crunch.
This was the worst part of the car, almost 200 fwkw and a cluncky and fussy gearbox.
Yesterday i changed my gearbox fluid. After reading on us forums and here i thought bugger it, I bought castrol vmx80 from supercheap and changed the oil.
WHY DIDN'T I DO THIS EARLIER!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I have a new gearbox! No WOT crunch or lockout! And the shifts, oh my god so smooth and alot less notchy. I'm actually starting to fall in love with the car again and all for $50.
So easy to do and the best mod i have done to the car by far!!!!
Change your gearbox oil!!!