new O2 sensor causing idling problems and misfiring

[saabmonster]

I have a 2003 9-5 aero. The check engine light recently turned on along with an error code for the O2 sensor (bank 1, sensor 1). At that time I didn't have any noticeable problems with the car. It ran fine and idled fine. But I went ahead and replaced the upstream O2 sensor (Bosch OE). I then used an OBD scanner to check for codes and cleared them. Two codes detected, listed below.

P0134 O2 sensor circuit - no activity detected - bank 1, sensor 1
P0300 Random/Multiple cylinder misfire detected

At this point I have a new O2 sensor installed, the check engine light is off and no codes appear on the scanner. However, it now has a problem idling. Each time I start the car it struggles to idle and right before it dies the engine revs and the tach surges. It sounds like it's misfiring as well. I haven't driven it since replacing the O2 sensor. I'm worried that it will die on me as soon as I pull up to a stoplight.

Should I replace the downstream O2 sensor? There is currently a brand new upstream sensor and a 15 year old downstream sensor. Not sure if that would cause this problem. Has anyone experienced this problem after installing a new sensor? Thanks.