Yeh - that was happening to me too. So I decided to just bite the bullet and use non-preemptible instances on my free credit which will obviously burn it up faster.
But even now I find sometimes it’s difficult to start a VM due to regions/zones not having enough resources. I had one running in us-west1-a last week, but could not restart it this morning. But was able to start a new one in us-west1-b. But it doesn’t have any of my work.
Trying to see if I can create a snapshot of the disk from the instance in us-west1-a to create a new disk for us-west1-b.
So I would recommend if you create your own notebooks on a GCP instance, then make sure you save them locally or something too.
I guess we are not alone: GCP preemptible instances seem unusable