Jeff:
Maybe the problem you describe with interference from other types of FM towers is what causes a sharp drop from a 4-bar signal to zero and back when watching Mason City channel 3.1+. Clear Lake S.P. is rather close to the transmitter, and other wanted transmitters are much farther away.
The picture will freeze or pixilate for a short period sometimes when camped there. Other trips no. Most likely due to some interference between us and the transmitter. This problem occurred with the old antenna. It is one of the reasons I find the Razor antenna very promising since I never had any problem when camped on the same site we prefer at the park.
I could routinely get the same channel from 100 to 120 miles away at many other campgrounds we have stayed at with no problems using the old antenna.