That's not enough data to accurately diagnose. However, my guess is the SWR is too high. I know you have checked this, but try a different SWR bridge. The reason I say this, most built in SWR bridges lack in quality and accuracy, and doubly so at the ends of their frequency coverage.
The ALS500's high SWR detect is rather sensitive on 160 which adds a little to the mix as well.
First thing I'd do is see what it does into a known good dummy load.
Alan, KØBG
www.k0bg.com
