Your tire is worn out. It just doesn't seem like it. If that thing has 500 or more miles on it, I don't care what it is, it needs to be replaced.
6 psi works fantastic. I do bump it up to 8psi when we go to the dez, but I ran 6 psi last weekend in a high speed area, with great traction with no funny business. The tire squirms at less than 6psi, but 6 seems to be the magic number. At least on my guage. (I have a 0-30psi guage)