Altitude display freezes

Hi there
I have a couple of PlayUAV OSDs on different planes and I love them.
I noticed today that the altitude display would freeze intermittently on a specific value before recovering 10 or 20 seconds later.
The "frozen" relative altitudes were all negative values as I was flying down from a ridge.
Not sure if this was a Pixhawk problem or a display problem.
Anyone ever see something similar?
Thanks very much.
John