I realize that this answer comes nearly two years after you posted, but I recently ran into the same issue with my sister's Samsung front loader. I'm posting this response as there are likely others having the same problem with their machines, especially as they age.
The machine would run a cycle, but would not spin up to full speed. It would get down to about 6 minutes, then bump back up to about 9 minutes and try the spin cycle again. I put the machine in service mode and there were no diagnostic codes that gave me clues. I also ran the spin test a few times and the machine spun up to 1200rpm each time (the spin test is done while the machine is empty). That told me that the motor was ok. Also, the inner drum was not physically loose - signifying that the spider mount at the back was not broken.
After a bit of searching around and watching some YouTube videos I determined that it may be worn shock abosorbers. The tub assembly is held up by two springs from the top of the machine, and is anchored at the bottom with 4 shock absorbers, one at each corner. I removed the shocks and they didn't seem obviously bad, maybe just a bit of loose play. I ordered 4 new ones from Amazon - just cheapies - didn't want to spend money if that wasn't the problem. The new ones didn't seem much better than the old ones, with some loose play to them too. I installed them, put the machine back together and ran it through a cycle with a small load. Worked properly after that!
I think what happens here is the machine starts spinning then with worn shocks the vibration isn't dampened enough, the machine detects the shaking as an unbalanced load and then resets the spin cycle to try again, it seems to do this indefinitely. The new shocks must be just a bit tighter and allowed the machine to get past the "shakey" part of the spin cycle to get up to full speed and complete. The new shocks were only about $40 for the set, compared to some that are 2 or 3 times that much so I'm not expecting longevity out of these, but will know what the problem is should they wear out again then can get more expensive/better quality ones to replace them.
The job to replace shocks isn't too bad. The back shocks are accessed fairly easily with the back panel on the machine off (two screws and the panel slides off). Apparently the front shocks can be accessed through the hole on the bottom of the machine but my sister's machine has the optional base on the bottom of the machine so I had to pull the front panel off. Not an overly difficult job.... pull the top panel off first, take out the soap dispenser tray, pull a couple of screws for the tray housing to slide it out of the way. The front control panel clips in at the top, and kind of slides up from the bottom with some other clips. Pull the electrical plug and put the control panel aside. Next pull the "ring" from around the front door gasket and pull the gasket off the front panel. Remove three screws that hold the door latch and lock onto the back of the front panel. There are 4 screws that hold the front panel from the top, and two on the bottom. The panel then slips forward and up. No need to take the door off. Once the panel is off, the front shocks are easy to get to and remove with basic tools - 13mm socket and ratchet. Be mindful of sharp metal edges when reaching in to remove and mount the shocks. I was able to complete the job in less than 3 hours. Can probably do it in two now that I know what I'm doing. LOL
