When i am driving my lighting and i hit about 55 my speedometer jumps really bad.. And then it causes it to downshift.. I have replaced the abs sensor and had the trans rebuilt and replaced the ring sensor gear and it still does it.. If anyone has any ideas of how to fix this or what to try please let me know.. Thanks alot..

 

i had same problem and did everythang you did and same resolts - i decide to rebiuld rear end and found out every thang was warn out replaced every thang and speedo and shift went back to normal