2004 XT. The dreaded P0011 and misfires.

I was just about to start taking things apart again when I thought I would ask you guys for some suggestions. I read so many threads on this topic but to be honest I'm not sure what to do next.

The Forester is a 2004 XT with ~165K miles. I bought at at 145K miles, changed the timing belt (was not replaced at 100K, can you believe it?), spark plugs, belts, coolant. The car was fine until earlier this year, when it developed P0021 and P0011 soon after an oil change (Mobil 1, 5W30, 5 qrts, and filter). The misfires were intermittent, and also did not happen every time I drove. When it misfired, the car would start idling rough coming to a stop light, but then it would recover. I read the forums and decided to take out all of the banjo bolt screen filters, including the one on the turbo. Surprisingly they looked pretty good, the screens were torn a bit, but it didn't seem like they would cause a blockage. I also tested the AVCS solenoids with a 9V battery and they worked.

I thought I was done, but 2 months later the rough idle returned. I drove the car 3-4 times around the neighborhood with the same results. Now when I start it when it's cold it seems to be ok at first, but after driving for 5 minutes it starts idling rough. I'm getting only P0011 with P0301 and P0303. The oil looks clean and the level is good (right at the top hole on the dipstick, though I haven't changed it after removing the screen filters). Not sure if this is of any importance, but after the first fix up until the P0011 showed up, most of the driving I did was with a loaded trailer which was definitely putting more stress on the engine.

Now I'm trying to decide what to do next. At first I thought maybe it's the turbo - I think it's still original from the factory and I do see oil in the intercooler. I heard some whining noise at some point, but it could be the alternator or power steering since I have no idea what a failing turbo sounds like. Still, this is my only guess. I'm tempted to try some easier things first - like changing oil. Next I was planning on taking another look at the AVCS solenoid on the passenger side. I haven't checked whether the timing belt skipped, since there's only 20K miles on it, but now I think I might need to.

Do you guys have any other ideas? What would you suggest I do first?