A balun is the matching transformer. Here's a offering from Radio Shack that's sure to be easily obtainable.
http://www.radioshack.com/product/in...ductId=2103912
If it's not water intrusion, then it's most likely that a tree has grown up near your LOS and you're now getting multi-path from it. Either selecting a more directional UHF antenna or relocating your existing antenna is likely to be the solution.
You do *NOT* need (or want) an amplifier - your signals are far too strong. If an installed amp doesn't go into intermodulation spasms from those strong signals, the tuner certainly will. Ignore that section of Denny's site as it doesn't apply to your location.