Remote troubleshooting offers some difficulties, due to the unknowns of previous operating conditions and prior maintenance and repair activities. It is also possible I am simply describing what you have alrady done.
Be sure to start the troubleshooting sequence from step 1, and not find yourself at step 99, realising that one of the preliminary steps was missed.
Start with a review of any recently accomplished replacement, maintenance or adjustment to systems directly or indirectly connected to the flight control system.
Next, the basics of your mechanical flight control system. (I am unfamiliar with any systems in the Mi-8, so ensure there is no reason stated in the Maintenance Manual to not perform these functional checks!)
This type of problem is consistent with worn spots on bearings and hardware in the control run.
Start with a mechanical check of the flight control run. (This check will run between servo's if you have more than one in the system.) Disconnect the flight control run at the servo input, it will now be possible to move the controls, downstream of the servo, through the range with little effort. Check for binding or flat spots. It is also worth disconnecting at the Cyclic and verifying the lateral cyclic moves normally. If there is more than one servo in the control run, repeat the process. This may allow you to isolate, with relative ease, which part of your control run is causing the problem.
Perform the following inspection on either an isolated part of the control run, or the entire control run if unable to isolate any problem.
Disconnect each section of the control run in turn and inspect the rod-ends, bell-cranks, bushings and hard-ware. Turn the bearings in each piece to ensure there are no flat spots. Ensure that no hardware is either bound in the bearings, bushings or supports. Ensure you systematically inspect EVERY part in the system. Check the bearings and housing supporting the lateral cyclic support, ensure these bearings are secured properly in the housing.
Inspect any mixing unit interconnects.
Verify rigging, stops, freedom and range of travel.
Many other possibilities, but start with these first. Keep us posted.