1995 Subaru Wagon engine shake, knock

I have a 1995 Subaru Legacy Wagon 2.2L AWD Auto. It is running a little bit rough:

putting sound each revolution coming from exhaust,
slight knock at higher RPMs,
small lack in power,
and the engine shakes side to side while idling.

These symptoms only happen when the engine is fully warm.
It has about 211000 miles now. When I put higher octane gas in, it is better for about 10 minutes, then reverts to the same symptoms. In the 3500 miles since I got the car I have:

replaced oil/filter twice,
replaced valve cover gaskets,
thermostat,
gave it seafoam (just before last oil change),
treated oil with that stuff for stopping smoking/blowby. It wasn't really smoking, just thought it might help.

What course of action would you recommend to fix this? Everything is in good condition including air filter, and recent timing belt job, and only very slight oil dampness on the valve covers. I don't know any history about the head gasket, but it is a reconstructed car within the last 3-4 years so it could have a rebuilt engine, who knows. None of the things I've done have made any difference in the shaking, knocking, or putting, although I seemed to have a slight bit more power after seafoam. I was losing about a quart of oil/1000mi, don't know yet if that part is better. Hoping to get a bit better mileage and a lot more miles out of the car, it's not too bad at 24-26 on highway but I know it could be better.