If it is doing this only after 15 minutes or so of being plugged in, it appears that it's possible it could be a heat-related issue.
Is this a tube mic? I'm not familiar with the model you specified, but my cousin has an MXL condenser that has a tube. The tube is located in a separate box and takes a 7-pin cable.
It * may * be possible the tube is going bad and the problem shows up when it gets decently heated up. Might be why the person on CL sold it.
If your model isn't a tube mic, I'm a bit stumped. I'm not sure the phantom power would generate enough heat to otherwise cause problems, but I guess it's possible.