2000 Subaru Outback Transmission

I have noticed over the past few weeks that the Subaru has been shifting hard and slowing in speed when driving on the interstate at around 80mph (It does happen at slower speeds, it just started at this speed). It does not do this all of the time. I can drive just fine then feel it pull back (or skip) and the speed slows a tad. Then as I go further the skipping gets stronger. When I get off of the interstate and stop at the stop sign its fine, until I go to pull off then it shifts hard, the RPM's rev up and the engine sounds like it will blow up and then it shifts very, very hard. If I stop the car and get out there is a distinct burning smell. The gas gauge is definitely affected by this, so much that when this happens I will burn more than a quarter of a tank in less than 25 miles. My soon to be ex-husband replaced the headers on this vehicle but this problem he refuses to fix or can't. He would rather me break down and have me call a tow truck with my kids. So I am left to figure this out on my own.