I would have the fuel pressure checked, I suspect a weak fuel pump or you got some bad (dirty or watery) gas.
Any other symptoms? Why don’t you make certain that all of the plug wires are on tight, plugs screwed in ok. This symptom (power loss on a hill) could be many things . . . start simple . . . look for the obvious stuff first. How many miles on the car? Last tune-up? Fuel filter changed when? Plugs/wires changed when? Timing adjusted correctly? How’s the idle? Start-up the same as before the power loss? Rocketman
This post was posted by accident before I finished it. You can find the full post a couple of posts down. My mechanic said that they’ve had several Subarus act like this and replacing the fuel filter didn’t work. Anyway… if you read the “real” post… there’s lots more info.