We revisit the problem of flow-induced structural changes in semidilute polystyrene/dioctyl phthalate (PS/DOP) solutions below their theta temperature by focusing on the high-shear-stress regime where thickening occurs. We observe a strong coupling of flow instabilities to the induced structure resulting from the enhanced concentration fluctuations. This behavior is manifested as a time-periodic signal of both the optical (birefringence and dichroism) and mechanical responses and represents an identifying signature of the strong-thickening region, being enhanced by shear stress and disappearing at low stresses or above the theta point. At the higher stresses, macroscopic rings are formed throughout the sample and sustained over the duration of the shearing. The emerging picture suggests that flow-enhanced fluctuations and eventually phase separation are the cause of shear thickening.