'95 Legacy, automatic transmission, high miles. It was running fine, then this problem suddenly started. It takes a long time to shift into gear, 5+ seconds to go from neutral to either R or D. It responds properly to gentle acceleration, but when the engine gets above 2000 RPM, the torque converter suddenly starts slipping. Can't give it enough throttle to get above about 20mph. We're hearing a buzzing noise at idle that wasn't there before, too.

Anyone have any ideas what might be going on?

You can drop the pan and spray the cartridge. You’ll see. The slider valves are exposed and I think you can even feel for binding. 
 

Try that first. 

I haven't had one of these transmissions apart before. What am I looking for exactly? And what am I spraying with?

Oil change/flush might help?

We did drain the pan and put in fresh ATF before we started driving it. Checked the level after the problem started and it still looks fresh, and about the right level on the dip stick. That's odd that it still looks fresh, the stuff we drained out was NOT fresh. I was expecting the 2/3 left in the torque converter and transmission to darken the fresh stuff.

Hey just so you know the advice I gave you was from a master Subaru tech who did this often. 
 

I also had to drop a pan and change out a full transmission harness etc. It’s not too big of a deal really. Good luck. 
 

Transmission pan gasket and some sealant if you desire. I think only sealant on the pan not to the block. That’s what I do on my old school pans. Never an issue. 

Thanks, moosens. We have a torque converter on the way, my son jumped the gun and ordered one. On to other projects for a bit until it arrives. We'll look up inside the trans when that gets here, and we drag the '95 back into the shop.