For those searching on this problem as I did...

Cold starting was hard; didn't even have to
be that cold.  Backfiring and poor drivability
until warm, then OK.

No check engine light.

Based on previous suggestions, I replaced the
coolant temperature sensor.  That completely
fixed it.  Good thing, too, I'm not sure what I
would have tried next.

BTW, getting to the thing is a b*tch.  I once
did it on my '95 impreza and couldn't remember just
how.  This article was most
helpful!  Step by step photos.

www.lovehorsepower.com/SubaruDocs/
CoolantTempSensor.html

Those hoses didn't just slip off, however!

Bill Hale Loveland CO