Transmission shudder

Shudders as you speak of -- specifically between 30 and 40 miles per hour in this case -- are possibly caused by torque converter or torque converter clutch issues. It can also be caused by fluid issues. Has the transmission fluid been changed as needed, and refilled to the proper specification Hands on inspection is needed. Seek guidance at a shop near you. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops