There are a total of no less than 6 static ports on that vintage of Grob. The 4 on the tail boom are connected together and there 2 on each side of the cockpit area as well. By design if any of these ports are clogged, a yaw will give a problem as you describe. Use a wire and probe the ports. You can blow carefully on each one and watch for a response on ASI.

By design a yaw will give some ram pressure to one side with a compensating negative on the opposite side , together should be close to true static.

Design must not have been great as it was replaced with a triple prob on the tail in the later models.

Buzz