This last post to subaruforester.org concerns a current, ongoing issue that is unresolved at the moment. There has been no uptake on the forum and I presume this reflects the age of the car and declining consumer-use deployment of the car model.
The prior posts were regarding issues that developed on the car, respectively, at about 168k and about 173k. The car now has a mileage of about 176k. In addition there were some major service events to the car that I had not posted to the forums which I do note here.

After Scanwest did such a great and reasonably-priced job with the alternator replacement I had them do the 180k service on the car at 175k, and at some point either before or after the alternator replacement but after AWD did the seals I had a cylinder lose pressure while driving on Aurora directly across from the indie shop Maddy’s Automotive. I was able to drive the bucking vehicle onto their lot and they fixed the valve overnight. I will be digging out paperwork on that job shortly and will provide more detail on it when I have the info in front of me.

Overall the car is aging gracefully and as can be seen we only put about 1000-1500 miles on it annually. It even passed emissions inspection this most recent time right off the bat for the first time since we have owned it.

On Monday I took it to Carter for an oil change, fluid topoff and an eyeball once-over. They did not report any leaks or oddities, as expected.

Today as I executed a lane change into the clear left turn lane on Aurora in preparation for leaving the arterial the car suddenly lost power for a moment or two and dropped in speed unexpectedly. Traffic was light and I was able to get into the turn lane by gunning the engine after which normal feel returned to the car. The loss of power was fleeting but it felt somewhat as if one of the wheels had lost traction, I think the front right wheel. However it was very brief and felt more like the engine had lost compression for a moment. There was no concurrent shimmy or bucking, which I did experience with the bad valve.

I didn’t think anything of it and went about my errands. About an hour later another errand came up and I hopped in the car. The total mileage for the trip was to be about a mile. I had gotten only about 300 yards from my house when a slight shimmy was felt when the car went into idle as I slowed for a stop sign and awaited the car ahead.

As I pulled into traffic, I felt the car responding sluggishly and responded by gunning the engine. At this time I noticed the check engine light illuminate and then intermittently illuminate and turn off. In general, gunning the engine spiked the rpms for a moment and then the car would respond normally for a bit before losing power again. Eventually this evened out and the check engine light stayed off and I made it to my destination with no further difficulty, although the car feels reduced in power and still has a new, slight vibration in idle.

Obviously, the place to start is at the dealer, since this issue seems like it could be from a loose hose. I wanted to drop this out here in case anyone else has any possible thoughts or concerns about a possible more serious issue. I have not looked under the hood yet, as I had some scheduled activities that could not be moved around and wanted to write all of this up first.

I’ll follow up with more info on that Maddy’s valve job and with whatever I learn from looking around the engine.

UPDATE

Visual inspection under the hood did not identify a loose connection, darn it.

Additionally, I was incorrect in specifying that I had a bad valve. The issue was a bad ignition point, one of the spark plugs was not firing. Maddy’s replaced the coil pack, ignition wire set, plugs and fuel filter. I think this was prior to the new alternator going in. That ran about $450 total.

I also have had one other major service event on the car I missed above not previously covered in other posts here. In December 2015 we had a very short, quite harsh cold snap that dropped temperatures rapidly from the 50s into the single digits and in the cold morning my radiator failed. I noticed it as soon as I returned from the drive and was able to drive it in to Carter and get it done in a day for about $500.

After talking to Carter on the phone, an unsatisfactory interaction in which the person I was speaking with seemed defensive and harried, they suggested taking it in to them to get a diagnostic code read, as the Check Engine light had illuminated. They noted that if they cold not attribute the engine fault to the oil change work performed last Monday, the diagnostic check would run about $120.

In looking into the way the car stores error codes, it became clear that a code reader was a trivially inexpensive piece of consumer car gear, starting at about $20 on Amazon, and so I think it may be wise to obtain on a fiend has one and is bringing it for me to borrow tonight, but either way I think the correct course of action is to perform a code read myself and examine the error to determine if it is attributable or not to the oil change. It does seem unlikely that a simple oil change would be performed in such a manner as to cause a loss-of-power issue but on the other hand, Carter was the last pair of hands under the hood.