[ADTech]

Both stations have on-channel boosters on Mt Diablo and might be causing the interference as suggested. A directional antenna with sharp null in the direction of Diablo when aimed at San Francisco would be needed. You'd be looking at two fairly large antennas to do that, though.

The other possibility is that, since you're so close to the numerous TV and FM transmitters on San Bruno and Sutro, is that the front end of your receiver is simply getting swamped. I'd try some attenuation (10-20 dB) at the receiver input to see if you can cool things off. Such attenuation would also knock the on-channel signals from Diablo down and would remove them as a possibility.

The other possibility is simply that you put the antenna in a multi-path prone location. Relocating the antenna, of course, is the remedy.