90 seconds is perfect for drain time on a top load washer. I would assume that you do not have any restrictions in your pump or drain line. If you clothes are coming out soaking wet sometimes, I would not think its the coupler, but I would look at the clutch assy. What happens is when the machine goes to spin, the centrifugal force pushes the inner assy of the clutch outwards against its housing. What happens over time is the pads of the clutch assy don't hold as well as they use to and it slips, allowing the tub not to spin as fast.
In order to get to the clutch, you must remove the pump, motor, and drop the transmission. On the top of the transmission sits the clutch assy. Its pretty easy to get to, not like a transmission in a car!
You can either replace the clutch, or sometimes with a dremel tool with a grinder bit, you can rough up the surface of the clutch housing where the pads meet, creating more surface for the pads to hold against.