Google Ranking Teetering In & Out Over Weekends - Bug Or Edge Of Quality?

Google Search seems to be ranking some websites just fine during the weekdays and then removing that ranking over the weekends. It seems to be focusing heavily on non-standard TLDs, you know, those new vanity TLDs that you can make up. I am just not sure if this is a Google Search issue or if the issue is with the quality of those websites....