I too have this problem. Its the drive belt that stick to the wheel housing. try some taculm powder. Mine starts to stick AFTER I dismantle it for washing (dish washer liquid). Usually after sometime on Autopilot, I need to "break-free" the sticky belt by jerking the wheel before its normal.
Aha! I've been searching all summer for an explanation of why our ST4000 drive unit binds periodically to jam the wheel when it should be free-running. I took the unit off the wheel, took it home and split the halves. Other than age, I could see nothing wrong, and the two halves run freely on the bench. But, on the boat, I had to rig a small line to pull up a bit on the drive motor to change the way the two unengaged halves relate to each other. That slight change allowed the unit to run free when the clutch was off.
Maybe its as simple as an old belt binding on the surface it is supposed to glide on? Perhaps it shrunk a bit in diameter over the years? Or perhaps the drive lever (clutch handle, I call it) flops down when the wheel is in the right position, clamping just enough on the belt to make it grab on the drive (pinion) sprocket?
After I posted this message the first time this morning, I found this in Raymarine's excellent self-help database, which I had not noticed before, dealing with binding and squealing: "The ST4000 and ST4000+ wheel drive units uses a toothed belt internally, which is tensioned by the clutch lever. The unit also has rollers internally which locate and guide the rotating drive ring. The ST4000 and ST4000+ wheel drives are BLACK in color. Do not confuse them with the newer ST4000+ Mk II wheel drive, which is GRAY. The MkII wheel drive has completely different internals. The parts and procedure listed in this solution do not apply to the ST4000+ Mk II wheel drive. Over a period of time, salt deposits can build up inside the unit and around the rollers and this can lead to the feeling of 'stiffness'. As time goes on, the rollers may also require lubrication. Any squealing noise you hear is probably from the rollers. Raymarine offers a wheel drive service kit, part number D169, which should restore the drive unit to like-new operation. The kit contains a custom tool for opening the wheel drive unit, a replacement belt, waterproof grease and instructions. You should avoid getting grease on the belt or the belt's running surfaces. You should also check the rollers in the clutch mechanism, they may be stiff too and may also need greasing."
UPDATE: Two marine suppliers told me the D169 repair kit for the ST4000 is no longer available.
By the way: I think the Allen heads are metric, which is why my 1/8" Allen wrenches seemed a hair too big? That's 0.125", or 3.2 mm.