Slice the pork into 1/2 inch thick along the grain. In an empty bowl, combine pork, soy sauce, garlic powder and flour.
Mix until the meat is well coated. Let it stand for a few minutes. Then put cornstarch in a deep plate then dip each pork and coat it thoroughly.
Heat cooking oil in a deep pan and fry the pork until golden brown, about 5 minutes. Then remove from the pan and transfer to colander to drain excess oil.
To make the sauce: combine ketchup, vinegar, brown sugar, salt, cornstarch and water. Stir until the sugar is dissolved.
Heat the pan with the cooking oil in high heat and refry the pork for 1 to 2 minutes to make it crunchier. Then remove from oil and set aside.
In an empty pan, heat 2 Tbsp. of oil and stir fry the veggies (bell peppers and onions) for 30 seconds. Then add the pineapple and stir fry for 30 seconds more. Remove from pan and set aside.
In the same pan, pour the sweet sour sauce and stir until the sauce is thick. Add the veggies and pork and toss until well coated with the sauce.
Turn off heat and transfer to a serving plate. Serve hot with cooked rice.