It does seems strange that only the 'Enter' key' would not work.
In general it may seems to be a power issue. You could test test the system out of the pc casing and see if its the same just to make sure there are short circuits happening.
You could try to update the bios or re-update (re apply the update) just to check if it was not any micro code error in bios.
Try clearing the cmos, remove the battery and put it back after about 15 minutes.
Best thing is to test with a different usb keyboard and see the behaviour.
Hope this is will be useful to you.