Au S1, Transmission Problem

[lostdude]

Hi guys, my au's transmission keeps going into Limp home mode after about 20 minutes of driving, where i only have 2nd and 3rd gear. I've topped up with fluid and it isn't low, transmission was serviced around 15000km ago.
I had the same problem about 2 years ago and i took it to ford, they diagnoised it as a broken solenoid.
It is probably the same problem again, as the car drives fine until the transmission heats up.
Has anyone replaced one of these before? what is involved, i'm ok with tools and prefer to fix this myself, i don't feel like paying ford another 500 bucks as im trading it for a new car next year and i really don't want to spend more money on it than i have to .

thanks in advance.