bizarre seasonal septic issue/sounds

Hey all - I'm back. It's crazy - I posted about this exact topic in late November 2010, 2011 - and now 2012.

Like clockwork, my basement toilet is "burping" when other water is used in the house. It's also a bit slow to flush.

In early 2012 (April) - I had the tank emptied for the first time since moving in. Drain field filter at tank end was perfect.

After this issue in 2010 and 2011, I had plumbers come out and tell me everything under the sun. And I did nothing. And nothing happened. Problem went away. I had the tank emptied in 2012 just because I knew it was time.

That now seems to have had no effect on this issue.

It's got to be seasonal and have something to do with the weather. I live in Atlanta, and it has gone below freezing just once for a short time at night. Otherwise I think it's been averaging in the 50s.

For this problem to come back - almost on the same day each year - I just can't figure it out.

I'm thinking it has nothing to do with the tank or fields. I'm thinking that perhaps it's my main drain line running to the tank. If it's colder - not frozen, but colder - perhaps that is slowing down the sludge trying to get to the tank and causing some sort of venting issue. Eventually it blasts itself through from the weight behind it.

Any idea?