Warbling vocals

Hi. I'm running LPX on an SSD iMac with an Apogee quartet. I'm occasionally noticing a kind of warbling distortion on vocals. At first I thought that flex pitch may have been the cause but I'm also getting the distortion on portions of unprocessed vocals. I'm using a Røde NT1 through a Focusrite pre amp, the same kit I've used for years on my old Yamaha 16 track with no issues at all. Any ideas?

Cheers, Richard.