Support

Hello David,

I suspect your server throws an error because it can't handle the requests that Kickstart is sending.
In this case the best thing to do is to locally extract your archive (using Kickstart or Akeeba eXtract) and then manually transfer all the files using FTP. Finally you can continue with the restoration as usual.
It will take a little longer but it's the best and safest way to complete the restore.

Backup was 399,000,000 3 days earlier it was 199,000,000 - wonder why since it is a fairly static site.

I suspect some big, large file got included inside the backup. I'd suggest you to take the two backups, extract locally and then compare them. If you're under Linux or Mac you can use the diff command, under Windows there's the WinMerge tool for this.

Is it possible that the restore will work better if using php 5.4 instead of 5.6 or 7.0?

No, that's not related.
When do you get the error? Immediately or after a couple of "ticks"?
In our experience, when a 500 error happens during the extraction, usually is caused the host that is blocking the request since it's using too many resources in a fixed amount of time. You can try to slow down the process: in Kickstart initial page, please click on the Show advanced options button in the Fine tune section.
There use the following settings:
Minimum execution time: 5 seconds
Maximum execution time: 3 seconds

Yes, minimum is greater than maximum, that's not a typo. That will slow down the extraction but it should complete.

ok, let me know if you need further help