06Tanner150

So for the past few months I've been chasing my tail on this speedo issue. The issue is I will be driving and I'll let off the throttle only over 40 and then the speedo drops and bounces all over the place. I read that it was the VSS on the rear pumpkin, replaced that and nothing. Then I read about the sensor on top of the tranny and that seemed to fix the problem for a day. Finally I followed the wiring and cleaned the grounds and that was a month ago, today the truck was fine but once again the speedo was bouncing. Sometimes at lights I go to start up and the trans seems lost and eventually slams into a gear, anybody know what else to do? Thanks