Pwave from .16 to .42?

Anyone ever see this before? We were kinda stooped at work yesterday on this one. Pt had normal sinus rhythm with a pwave interval of .16. Then, went to a pwave interval of .42. Mind you, we realized this was a first degree block. But, the pt kept going in and out of this rhythm a few times. Eventually, pt converted back to normal sinus rhythm with the pwave at .16. We asked cardiologist about it. Couldn't really figure out what he said. Was this just a fluke thing? This was only showing up in one lead. Checked all leads, they were good. The Pwave was not inverted. Otherwise I would have gone with something like a junctional rhythm. Anyone familiar with this? I'm just curious because I have never seen this type thing happen before.