After some instensive digging around various Subaru fora and borrowing an OBDC II code reader from a friend, I still don’t have a good read on the car. The code reader did not show any codes recorded, which seems odd, but whatever.
Fora posts describe my car’s symptoms in similar cars but not the ’99 Forester. The consensus appears to be that transient loss of power accompanied by a front end shimmy is a cylinder misfire, likely due to a bad plug, a bad wire to the plug, or imminent failure of other associated parts. Which, great. Except I have actually experienced this and had all that both replaced and serviced within the last 3k. Furthermore, the shimmy I was experiencing was much less intense than that I had when the cap was failing.
So possibly that indicates incomplete chamber combustion rather than total non-combustion?
Another possibility, which seems somewhat more likely as it has a possible causal tie to the recent oil change, is that Carter overfilled the oil reservoir. A few posts here and there (interestingly, the most informative one was from a Seattle-area owner who had taken his Outback to “the dealer,” presumably Carter, for an oil change) indicated that oil overflow into parts of the engine that did not need it could lead to power loss, and that this overflow could occur if the oil reservoir was overfilled. The dip stick is said to have fill marks for hot and for cold weather, the cold weather mark indicating a greater acceptable quantity of oil.
Persons discussing this issue and possibility indicated that burning oil, blue smoke, would be expected to accompany the issue. No smoke has been noted. However, when I checked the oil level this morning it clearly exceeded the highest fill dot level. So presumably they did overfill the car. However others also note that moderate overfills are used in some situations to provide a safety buffer for a car in extreme situations, and this past three weeks might kinda qualify from a temperature perspective, so who knows.
The car had two runs today, each about three miles, and no shimmy or loss of power was resultant.
UPDATE: After subsequent power loss and a diagnostic visit to Scanwest on Greenwood, it was suggested that the ignition coil might be failing. The replacement was within warranty horizon, so I toot it to the initial shop that replaced it, Maddy’s on Aurora, and they verified the issue and replaced the coil without incident.