I have a Craftsman 15.5 lawn tractor Model No. 917.271024. The transmission demonstrates a violent jerk either when moving control lever forward or backward. Checked motion drive belt (good), cleaned transmission fan blades and transmission fins, adjusted motion control lever to stop the creeping forward and backward problem. When shifting speeds, helps to hold control-lever close to side of control-lever guide, slowly. Manual says not to change transaxle pump fluid due to factory sealed. What can I adjust or replace to solve problem?