I believe what you might be seeing with the voltage changes is the computer trying to fine tune the A/F mixture. It will continuously cycle between slightly rich and slightly lean, trying to maintain A/F at 14.7:1. Normally it doesn't swing that much in either direction though.
At least you know the O2 sensors are working correctly - voltage should drop if the motor is running rich.
Does it still do this after it's warmed up? You may need to run it for a while after disconnecting the battery for the ECM to figure everything out. I'd also double check all vacuum connections.
Does it run fine otherwise? Any codes?
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