If you look at previous posts on the issue, it surely sounds like something dead. Sometimes air infiltration from outside wind or air from a fan will cause the smell to be stronger on some days. If it can be found and removed I'm sure the smell can be taken care of with good ventilation and commercial smell removers.

Let me ask you this--what do you heat with? Do you heat with propane?

For some reason, the odor they put in LP can smell like a dead animal. I had this same problem in my house until I finally realized I had a pinhole leak in the gas line going to the dryer!!!!!

Are your drain vents galvanize pipe? My guess is you have a separation in a fitting (possibly a lead slinger if used) in one of your vertical vents. A really, really, really, experienced plumber can find this out for you. BTW I have found 2 failed slingers in my 21 years.

Is your roof top sewer vent leaking in the attic? Does your fan vent exit on the roof? IF it does, do you have a filter on your roof top sewer vent? Check the floor seals on the tolets! If they leak the sewer gas will come into the room. Check my Ezine Articles for more imformation.