Got my replacement today. Same problem after one hour.
I don't think the replacements are going to help much, unless you get lucky because the issue has not been worked out by Apple. It likely needs a firmware update like the original aluminum macs needed (I was lucky not to get a bad one back then, not so lucky now with all the Air issues I am having though).
Are you calling them right now? Because you really should.
Like the other thread says, it could very well be the WiFi (you're using wifi, right?). Apple NEEDS to know this. not necessarily that you're using wifi (okay, maybe so), but that there is a SERIOUS problem with the 4850s. If you're allowed to guess, tell them.
Ask them if they're aware.
Tell them it's in the news (appleinsider).
They will sound clueless, but they really need to be aware of this problem.......
What is wrong with the 4850s? That is a card you can upgrade in your iMac configuration purchase right?
Why?
OOO I know lol jk. Didn't apple test them before they shipped?
Looks like they didnt.