Hello John.
The symptoms you describe are classic for cars that are running just
a wee-bit lean. I have little knowledge about the Subaru engines but
this is typical of cars without computer control that have
had "performance" exhausts put on without increasing the fuel/air
(ie. jet changes for us old carb types).
I would suggest that perhaps the O2 sensor is suspect here since even
if your exhaust was incorrect, the suby computer, with input from the
O2 sensor, should be able to handle any variations in back pressure.
Good luck,
Mike Pidcoe
