I've had this on a few of my subaru's and I've never managed to find out what the problem is. Only seems to happen after I've been driving around or a few hours, you pull away and it's sluggish until you get to about 3rd gear and the power seems to come back again. Checked the oil, water, plugs, fuel filter air filter, oil filter, O2 sensor, idle control valve, everything I can think of but I still get it from time to time. I've had it on turbo cars and non-turbo cars aswell