car has a shudder

The shudder -- is it momentary or continual? Where do you feel it? How does it stop?

Additional details needed! Have knowledgeable mechanic test drive vehicle so this concern can be experienced first hand. Mechanic may choose to have scan tool connected during test drive - - particularly to monitor ignition system for possible misfires. Flashing / blinking check engine light is one telltale. Good luck.