why scrapebox proxies connection not stable?

Whenever I'm checking proxies connection i'm getting some good proxies, it's tested. if i'm testing again the proxies, it's showing some proxies filled, but previews it has showed pass. i don't why the scrapebox proxies is not stable.

How can i slow this one ? If any one knows plz help me

This is the nature of free proxies found on the internet unfortunately. These proxies can be low powered servers or home connections with limited connection speed. When posted online, they may have hundreds of people trying to connect to them with some people trying to run hundreds of connections through the one proxy. So they can go offline and come back online as people start and stop using them.