I'm not sure what has happened with your previous attempts. Are you using rice vinegar or another type? Check your soy sauce alone and see how it tastes.
This recipe tastes very similar to PF Chang's Sweet and Sour Sauce (though it does include the ingredients you mentioned).
1/4 cup water
2-1/2 Tbs unseasoned rice vinegar
2-1/2 Tbs sugar
1 Tbs ketchup
1 tsp soy sauce
1/8 tsp salt
Dash red pepper flakes (optional)
1 Tbs corn or peanut oil
2 tsp minced garlic
1 tsp minced ginger
1 tsp cornstarch dissolved in 1 Tbs cold water
Mix together the 1/4 cup water, rice vinegar, sugar, ketchup, soy sauce, salt and red pepper flakes and set aside.
In a small saucepan, heat the oil. Stir in the garlic and ginger and cook until they soften; do not let them brown. (You may need to turn the heat down so they slowly sizzle.)
Stir in the liquid seasonings (except for the cornstarch mixture). Let the mixture come to a boil. Reduce the heat and simmer for about 1 minute, stirring, to allow the flavors to blend. Stir in the cornstarch mixture and heat until the sauce is glossy and tastes cooked through. Turn off heat and cover to keep warm. Sauce will keep refrigerated several days. Serve warm (reheat if necessary).