Engine loses power and sometimes stalls

I got a 93 Subaru Legacy AWD, 2.2L(non-turbo), 150k, from a tow yard. It sat for at least a few months. It needed a new clutch so i put one in myself. After driving for a week or so, i noticed that the engine seems to idle erratically sometimes. It runs perfectly fine when the engine is cold, but once it gets warm the RPM's move around. Also, when driving in the higher RPM's, sometimes it will lose power or have little power. It has even stalled a few times.

I changed the spark plugs, pcv valve, and the fuel filter after having this problem for about a month and many tanks of gas, so i dont think it's bad gas. I ran a can of Seafoam through the vacuum lines and the gas tank. That made it run better for about a day. I ran 91 octane gas, and that made it better for about 50 miles.

Anyone have any ideas of what could be causing this?

Thanks