Can't install High Sierra, and other problems

my 2011 imac running latest version of sierra before high sierra started acting up: slowing crawl, beachballing, low quality video streaming. tried reinstalling os internet recovery didn't work – log showed error messages connection being "invalidated."

took apple store , reinstalled os, tested hardware etc. , said checked out. got home , same problems started again. took local authorized repair shop , had them put in new ssd. restored time machine must have contained whatever caused problem in first place started on again.

since i've been trying erase disk , install os scratch, thinking i'd transfer files icloud. internet recovery still not work. tried making bootable usb drive, , did, once, allow me wipe disk , install high sierra. sorts of other things went wrong. instance, in mail, when click on preferences, nothing happens – no window @ all. looked in library , there's no mail plist, it's impossible make new 1 removing current one. when try reinstall usb drive, freezes, or won't connect mouse, or creates other problem.

internet recovery can't wireless problem, because tried on computer in house , worked fine.

thoughts?

 

one of internet speedtests e.g this 1 may give datapoint. compare other computer.

 

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