Wireless Ethernet Bridge

I have a YDI (now Terabeam) Bridge-in-a-Box linking two LANs about 1 mile apart. The units are mounted outside. Since installation, the link will drop randomly for anywhere from 10 minutes up to several hours and then come back up. I have worked with tech support and tried their suggestions with no luck. It has dropped in all weather including bright, sun shining days. The one site is at a higher elevation than the other. Any ideas?

Thanks,
Steve