It appears to me that the original bearings on this era of frames from Japan are the root cause of the "hinge in the middle" analogy often quoted disparigingly about them.
If there's much difference in one shock absorber to the other (fairly easy to get) then with fast compression it starts to twist the arm severely enough to pinch the pivot tube bearing alignment through torsional distortion
effectively seizing the arm and causing significant displacement of the rear wheel track relative to the front. Hence that going in two directions at once feel the rider gets especially when you meet a dip mid corner at any significant pace.
Quite possibly one of the best improvements you can make to the original frame is to change these bearings to roller/taper/bronze type that won't pinch like the original materials.
