I've had mine do this once when I started it on a cold day, backed it out of the garage and let it idle in the driveway to warm up as I shoveled the walk and after a few minutes of idling the air pump came back on. It was the first time I noticed it but I came to the conclusion that the cat had cooled off since I wasn't driving it and it was very cold outside. My understanding is that the air pump helps get the cat up to temp on a cold engine. So.... that being said, it's possible your O2 sensor is going bad and giving false readings to the ECM so it turns on the air pump because it thinks the cat is not up to temp OR, your cat is actually going bad. At least that's the determination I came too but I haven't proven any of it.

DEWFPO
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1998 S90 083,228 and 1995 964 154,100