Pitch Instability at high airspeeds

Has anyone noticed a pitch instability at high airspeeds with foam planes? I’m wondering if it’s more a function of the airframe itself or the scaling of the PID values. I normally cruise and autotune around 15m/s, but when I push beyond 30 m/s, the airframe begins to pitch oscillate a little slower than 1 Hz.

The pitch tune is solid for lower airspeeds. Roll is good at all airspeeds.