Trim fat from steak. Cut steak into 1 inch cubes. Place steak cubes in a resealable plastic bag set in a shallow dish.
For marinade, in a bowl combine salad dressing, thyme, Worcestershire sauce, and garlic powder. Pour over steak; seal bag. Marinate in the refrigerator for 4 to 6 hours, turning bag occasionally.
Drain steak, reserving marinade. On eight 10 inch metal skewers, alternately thread steak, sweet pepper, red onion, and potato, leaving a ¼ inch space between pieces.
For a charcoal grill, grill kabobs on the rack of an uncovered grill directly over medium coals until meat reaches desired doneness, turning once and brushing occasionally with reserved marinade up to the last 5 minutes of grilling.
Allow 12 - 14 minutes for medium doneness (160°F). Discard any remaining marinade.