In a large bowl, combine the first six ingredients. Pour half of the marinade into a large resealable plastic bag; add beef. Seal bag and turn to coat; refrigerate for 8 hours or overnight, turning occasionally. Cover and refrigerate remaining marinade.
Drain and discard. On metal or soaked wooden skewers, alternately thread the beef, shrimp, mushrooms, green peppers, onions and tomatoes; set aside. In a small saucepan, combine cornstarch and reserved marinade until smooth. Bring to a boil; cook and stir for 1-2 minutes or until sauce is thickened.
Using long-handled tongs, moisten a paper towel with cooking oil and lightly coat the grill rack. Prepare grill for indirect heat, using a drip pan. Place kabobs over drip pan and grill, covered, over indirect medium heat for 6 minutes, turning once. Baste with sauce. Continue turning and basting for 8-10 minutes or until shrimp turn pink and beef reaches desired doneness. Yield: 6-8 servings.