1998 Hyundai Accent Hyundai loses power when driving up hil

I had changed the converter 8 months ago, and car has been great since, although I have been putting additives in to control the oil burning, and smoking. The car drives fine on the level roads, once the road inclines, the car slows, and I have to accelerate to try to maintain speed, if the hill is to high, I generally drive in second gear, but not near posted speeds. I was wondering if the fuel filter or pump might be the problem. It starts great, idles fine, and runs good. Could the excessive additives cause the converter to fail again? Are there any checks that I could perform myself to isolate the problem? Thanks for your time. Ralph