I have a Trek 8000 with XT Group set. I am getting a horrible gear slip and crunch in certain gears. If i put the power down in either the front large or middle chain ring and either cog 8 or 9 at the back the gear slips and there is a loud metalic clunk and a momentary lose of power, before it then picks up. I cant see any excessive chainring wear and i have fitted a new chain. My guess it is the freewheel, but before i blow cash changing it, i wanted to get some opinions as it is very hard to pin point.

Cheers

Sounds like you just need your gears setting up that's all. Take it to your local bike shop or do it yourself. If it's still slipping after that then you may need a new cassette, especially if you have changed your chain as the pair tend to wear together.

I think Dan98 may be right – slipping freehubs do pretty much as you describe