When driving 70 mph my car seems to down shift an rpms jump why.

Hello, it sounds like for some reason the automatic transmission is losing engagement when engaged in the highest gear. If the transmission loses engagement, the engine speed will rev higher and you will lose speed and power. Hands on testing utilizing a diagnostic tool to monitor transmission operation is needed. Checking for any present diagnostic trouble codes will be a good start also. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops