I had two power sweeps in my tank for five months and they never stopped moving back and forth. I heard that they were redesigned and work much better than they used to.
I only stopped using them because I added a sump/fuge on my tank with a SCWD attached to the return line to give me some good alternating water currents. The thing has worked great and now the only equipment in my display tank are my overflow and the two pvc pipes on each side of the tank pumping water out.