It looks like we're hitting the daily quota every 5 days exactly due to a regularly timed spike. On April 26, May 1, May 6, and May 11, there were huge spikes in Google Search API usage from Tool Forge resulting in hitting the quota and then being denied service for the rest of the day. The latest spike was from about 1am to 2am (PST) on May 11. This is most likely coming from Earwig's Copyvio Tool (or its API), but we should do some logging and check.
API documentation: https://wikitech.wikimedia.org/wiki/Nova_Resource:Google-api-proxy