I doubt your machine has worn out. Seriously.
Step 1: strip it down, top and bottom, and clean it out, then oil it. Check for really free running with no needle or thread.
Step 2: go over all the tension adjustments and get them right. You may need a manual. Check the web for this.
Ah - is it an older mechanical one or a modern one with plastic gears, stepper motors and electronics? The modern ones can lose a sync sensor - which sometimes can be fixed.