Engine overheating

I have a 1998 Suzuki Esteem Wagon. I recently put a new oil pump in it & now it is overheating. I found a leak in the water pipe that connects to the engine, so I replaced the o ring and siliconed that. Then it still was overheating after driving only a couple of miles. I checked the thermostat & it is functioning properly and the water/coolant is flowing like it is supposed to. I know the water pump is working so its not that. My fan is running properly as well. I am having loss of power when I start to move, but once I am moving it seems to be fine, but the check engine light is blinking and its still overheating very quickly. I can only go a couple of miles before its in the red zone again. I can only think the cat. Converter might be stopped up. If its not that what else could it be?