Engine Troubles - Going to a new Mechanic Shop

It was probably a month after I owned the the forester that I decided to take it in to get the oil changed.  The previous owner said he changed the oil about every six months, I was shocked at this time-based form of scheduling.  I try to make sure the the oil gets changed in my vehicles every 5-6000km based on the kind of driving involved.  I go to a quick lube shop that is owned by someone I once had as student so I like to support her business.  The service technicians noted that there was antifreeze on the engine cover but I had just topped up the fluid and spilled a little in the process.  I made a note of their comments and paid the bill.  In talking with the technician they mentioned the garage next door was run by a former subaru expert and that they were a good place to go for my foreign car.

The car had been performing poorly for a couple of weeks but in sporadic and unpredictable ways.  I was monitoring the situation and noticed that the engine hesitated for a while after I first stated to drive it down the road, then by pressing on the accelerator it would eventually clear often only taking a few seconds.  I initially thought it could be a bad fuel filter, so I picked and replaced the fuel filter and air filter and the car ran better, but hesitation kept coming back.  Then one day on the way to drop off my partner at work the call really started to act-up.  The engine skipped a few times and the check engine light started to flash.  It didn't stall but it was close, I felt like it was all I could do to get it to climb the hill.
I took it over to new mechanics that I had heard about and asked them to take a look at it and fix it.  They took it for a test drive and recommended that wire and plugs needed to be changed.  They also read the computer codes and said the same thing about the sensors the other mechanics told me, the O2 sensor needed be replaced.  They cleared the codes and changed the ignition wires and plugs and it started and ran much better costing me about $250.  Totally worth it.

Location of the spark plugs on the engine.

The next day the hesitation was still present but not in the same way it was before often at 2000 rpm and when the car was shifting gears.  I call the shop and asked if I could drop in to have the new codes read and they said no problem just come in.  Based on the new codes the mechanics diagnosed that the issue was the knock sensor and that it should be replaced.  They had mentioned it before but weren't sure it was necessary, this time they were. 
I asked if they could 'slip' me in to put on the $135 sensor that morning and they had it ready in less than an hour, and cost a half hour labour charge.  I was very happy with the service and impressed with their honest billing and assistance. The car has driven like a dream since then.  It seems like my gas mileage has gone down a little but if the car was running under hesitation before this makes sense.  The exchange for a little gas mileage was increased power, speed and dependability.

Knock Sensor, a little sensor with an important roll.