Car sputters when I hit 40mph. Ok after 50 mph. 2008 mercury marqui.

Classic symptoms of misfiring ignition coil at a particular engine load / rpm.. You’ll need a professional grade scan tool connected while operating vehicle so live cylinder contribution data can be observed and misfiring coil can be identified. Likely will need *hands-on* assistance from qualified personnel. Good luck!

Second thought could be transmission torque converter shudder - just depends on what it feels like and what level of acceleration is requested by the driver. A local technician near you can point you in the right direction, and better yet diagnose and repair the vehicle.