Just built another new computer but was wondering the antec fan on the back of the ps turns itself off o rpm and then turns itself on changes fan speeds all the time is this normal?

The fan doesn't really shut-off, it just goes to a lower RPM than the sensor can detect, they stop detecting around 1200-1300rpm. If you're really worried about it, look at the fan and verify that it still works.

Fan speeds tend to jump around a bit too, because the load on the unit is always changing. As long the RPM isn't wildly jumping around everywhere, I wouldn't consider it a problem.