I am having problems with the power steering. It stiffens up, and then works fine quite frequently. I checked the fluid and it's very dark and gritty. Any ideas to what is wrong?

I know this is an old thread, but I didn't see it earlier.
Your steering rack has 'morning sickness' Its likely stiffer when the car is cold and loosens up as you drive, when things warm up.
The rack is severely damaged inside and is not repairable. The dark colour of the fluid is likely from fine aluminum particles.
You need a new rack. The PS pump should be replaced, too, or at the very least flushed, or else the old fluid will contaminate the new rack