Parts Avenue

1991 Subaru Legacy 4 cyl, 2.2L AWD,  Manua,l 170,00 miles: 

When the weather is below about 45 degrees outside, my Legacy won't idle smoothly or reliably. Instead, it surges between 0 RPM and about 1,300 RPM, up and down, until it stalls. The colder it is outside, the worse the problem. 

When I take my foot off the accelerator while driving, if the outside temp is in the 20s or 30s (or lower), idle drops to nothing and the engine stalls. 

Both of these related behaviors happen while the engine is warm and while the engine is  cold, though at startup wihen the engine is cold, it actually will run smoothly for about 20 to 30 seconds before the surging begins. 

I replaced all vacuum lines, cannister, mass airflow sensor, throttle position sensor, purge solenoid control valve. No change, except that I no longer get a diagnostic code indicating that the purge solenoid control valve is stuck open. 

Since that is apparently NOT the problem, I'm hoping someone will recognize it. It is definitely tied to outside air temperature. On those rare wonderufl winter days here when it's 50 or 60 degrees out, the car runs perfectly smoothly, as it should, no idle issues at all. Soon as the outside temps drop, the surging at idle issue begins. 

Suggestions?