I bought a Pixel 2 a few weeks ago and upgraded it to Android Pie, but about every three days it comes up with a notification asking me to restart (I forget the exact wording of the message). Have done this about five times now. Any idea why this is happening and if there is a fix?
It could be just your regular OTA updates. Pixel phones receive updates every month. And sometimes there can be more than 1 update in a month. These are monthly security patches which are delivered and installed to your phone automatically. The device then needs a reboot to finish installing the update. Depending on what software the device shipped from the factory, you'll receive several updates to come to the current build.
For example, if your phone shipped with the September 2018 security patch, the device will install October, November, December, January and February before resting on the latest release. After that it will be one security patch update every month.
The updates are incremental. So, you'll have to install all updates one after another. You cannot skip directly from September to Feburary unless Google pushed the full packages each month. The Pie update was pushed in August 2018. So it will be with August security patch. Now your device needs to update from August to Feb. I believe it should be almost done, if not, a bunch of more updates will finish the backlog.
You can probably finish all updates one after another by checking in Settings > System > Advanced > System update after each reboot to ensure that you have finished installing all available updates.