sometimes the download will get to 75% for example and then time out or have to wait so many minutes or error.
when it starts again sometimes it will go from 75% which makes sense but sometimes it starts all over again at 0% and will have to download that 75% all over again. why does it start at 0% sometimes?

Connection or server problem.

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Most servers don't support resume (they want you to buy a premium account), that's why it sometimes starts from 0%.