You're in the south bay aren't you? Conditions affecting Mt. Allison are likely to affect you too.
A friend in Placerville, who is quite some distance from here, has trouble with the Walnut Grove stations (not necessarily the same stations) at the same time I do almost every time. A given set of conditions can affect a wide area.
It's impossible for the ERP to drop in one direction and not another with an omni directional antenna. It's impossible for the ERP to drop in one direction and not another with a directional antenna unless the antenna is rotated.
If a transmitter antenna was having a problem with a portion of the elements in the antenna that would be noticed at the transmitter because they monitor many parameters of the transmitter including VSWR for each transmitter rack.
So far, the only problem I can remember that has popped up on this forum that is attributable to the station and unrecognized by them was the KGO RF 35 issue. And that's a problem with reception, not the transmitter.
It's very common for one station to be more affected by atmospheric conditions (temperature inversions) than other stations at any receive location. I have that issue with KCRA and my Placerville friend has it with another station. There's no easy explanation why that's so but I have noticed that the same problem tends to repeat. I also see the same enhanced conditions repeat such as booming signals from Salinas. If you have trouble with KGO 7 at the same time that Mt. Allison has trouble then it's just a coincidence of your locations and the tendency for reception issues to be repetitive. Remember, correlation is not causation. Just because two locations experience a similar signal variation doesn't mean it's the transmitter.
Edit: Add clarification
Edited by Calaveras - 7/24/12 at 12:49pm