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, onion, and potato wedges, leaving a 1/4-inch space between pieces.
For a charcoal grill, grill kabobs on the rack of an uncovered grill directly over medium coals until meat is desired doneness, turning once and brushing occasionally with reserved marinade up to the last 5 minutes of grilling. Allow 12 to 14 minutes for medium doneness (160 degrees F). (For a gas grill, preheat grill. Reduce heat to medium. Place kabobs on grill rack over heat. Cover and grill as above.) Discard any remaining marinade. Makes 8 kabobs, 4 servings.