1998 Subaru Legacy 1998 Subaru Misses Going Uphill

My 1998 Subaru does fine until it goes up any type of hill. It then starts missing and almost feels like it is choking down.

Turning on the air conditioner makes it worse.

I have checked the air filter, fuel filter, and had new head gaskets put on it last year. It was leaking cooling fluid but I found the leaking hose and it has been replaced solving the problem. Also, it seems to be loosing a bit of oil.

What would be the first thing to do? Plugs and wires come to mind but I cannot figure out how to get the plugs out seeing they are so close to the frame.