Chosen Solution
I own a repair shop I have a 21.5 inch late 2012 iMac is running so slow it times out installation of OS, and is jumpy during navigation of installation media. New Ram old ram same issue, new SSD old HDD same issue, new thermal paste same issue, any ideas are much appreciated.
I think the original problem was a bad HDD was causing a similar issue as the lagging and jumpiness, but after I installed the SSD I never reinstalled the LCD I only tested without it, so it seemed like it was the same issue but it was a sensor not plugged in.
