I've been having some issues with my late high end fusion drive 21.5 inch.
The problem first occurred 2 month after i bought the iMac. Was having really really slow wifi speeds, although the signal strength was full. I called my internet supplier who confirmed that everything was ok, after he had started a monitoring of our connection. He even told me that he'd never experienced any problem with the kind of router we have, when pairen up with Apple products.
I called Apple tech support after the problem periodically showed up roughly every second day. Was measuring speeds of 500 kbit download while my macbook pro would make it to 7-8 mbit when standing right beside my iMac.
We tried some different things, but nothing seemed to be working. He's last recommendation was to hand it in at an apple store. Although I could end up with paying 500-600 kr (80-110$) if the fault in the machine couldn't be recreated at the workshop.
The tech supporter's final excuse, IF the wifi on the machine wasn't defect, was that "the machine was in a bad spot with too much wireless interference".. But every other device works perfectly there.
So status is right now I have a really really unstable machine. Fortunately the problem haven't occurred since I called the rep., which have been one week now.