Vent pipe fumes

We have a brand new home (2 story with 3 baths). Weve been living in the house for 1.5 months. As of about a month ago, weve noticed a strong (at times) septic smell on the opposite side of our house from the septic tank and bed. The smell is only outside and is at its worst when the air is still, cool and damp. We live on the side of a hill with no trees close by. Our kitchen sink is not hooked up presently but the pipe is capped where it sticks out from the wall. I did take the cap off last night to see if there was a strong odor in that pipe… there was no smell that I could notice. At the same time, the odor was outside the house at the back and high side (same side as the vent pipe). We have an 8/12 pitch on our roof and the vent pipe which sticks up about 1.5 is far below the top of the pitch. Weve been told to extend the vent pipe over the height of the pitch. I doubt this will work, as the smell is at its worst when there is no wind to take it anywhere plus the vent pipe is already up 2 and a half stories. The smell would still drift downwards to the ground on cool, damp nights. All our toilets and drains work properly and there is no odor in the house unless a window is open and the smell from outside blows in. We have a roughed in bathroom in the basement with just the piping installed for now (all piping there is also capped). The septic tank is about 15 – 20 feet away from the house and we have a 150 disposal field (we have never smelt an odor around either). If we leave the house for the day, or if its first thing in the morning (before everyone is up and using the water), there is no smell outside… The smell only becomes noticable when the house is being lived in. I dont know if Im grasping at straws here, but… the attic door is not installed yet in our house… when you stand under it, there is almost a nice breeze that comes down from the attic into one of the bedrooms (no odor has ever come through though). My question?? Could this be affecting the way the air around the outside of the house is moving? Maybe the attic vents are pulling air in that would normally blow past and over the vent pipe therefore taking the stink away. Were going to seal that attic opening this weekend anyway, but at this point Im trying to find reasons why and a simple solution to this awful smell. Some people have suggested that maybe since the septic system is new, that the top crust in the tank hasnt formed well enough to trap the odors yet…. but then, I would presume all new houses with septics would have this smell too and that doesnt seem to be the case. I really dont want to break the concrete floor in the basement to put in a gooseneck or some kind of trap to keep most of the odor out of the vent pipe but I guess Ill have to if I cant find a better solution. Im just looking for advice as to what has worked for others in this same type of situation. Help!