This has been happening for a few months now, it comes and goes.
On wide open throttle the car will rev to about 4000rpm and it will just lose power and not change gears (automatic by the way).
Even in tiptronic, it will rev hard like it should, and it'll reach 4000rpm and just lose power, then i'll shift it up a gear myself and it just doesn't pull like it should.
Other times it'll pull hard, right up to 5800rpm like it should and change gears automatically.
It makes over taking hard on a freeway sometimes, as when I'm driving, i'll put my foot down, and the car will drop from 5th to i think 3rd gear, and then will lose and can't get around someone.

Has anyone had this problem before or have any idea what might be going on?
I'm thinking maybe a trans flush might help. Not sure.

Well coming from the Skyline scene, this problem occurs way too often. Some things to try and check before you take it to a mechanic. Check spark plugs (more often than not, these are stuffed), check coil packs to see if theres cracks in it - even a fine hair piece crack will cause your 4K Rpm issue, clean your AFM with a decent cleaner, Don't think its your fuel pump, Could be your cat (but highly unlikely). See how you go with these checks.

Good call jattsta. Coming from the coil on plug system of the useless falcons ( B series ) this to is where i would start to have a poke around.