fuel injectors leaking on a4

[rootie 17]

Hi I have a 98 A4 wagon v6. I'm having a problem with the upper o-rings on the injectors leaking whenever the air temperature is cold. This never happened in the summer. I replaced the o-rings on the passenger side 3 injectors and after a few days they just randomly began leaking again. Today the temp was 23 F. and when I started the car, three injectors were leaking badly, 2 were ones that I had replaced the o-rings in about two weeks ago. As I said, it doesn't happen in warm weather, only when the temp is colder than about 40 F.
Has anyone experienced this? I guess I could replace the rings in the other 3 injectors but I don't understand why the ones I already replaced are still leaking. Please give me your suggestions. Thanks everybody.