Glowing catalytic converter and fuel problem

My son has a 1999 Hyundai elantra that started acting really sluggish about a month ago. I noticed it had an exhaust leak and we looked at the front cat ( they have 2) and it was glowing red hot. The flexible exhaust pipe connected to it was glowing red hot and torn in half, hence the leak. First thought was back pressure from lower cat. Had to replace upper car and flex. Did that and started car. Car started fine and sounded great for about 5 minutes. Noticed lots of white exhaust and fuel smell. Looked at new cat and of course it was glowing red hot. Looked at spark plugs. Two of them looked fouled with strong gas smell. Changed those and fired car up. Again sounded great for 5 minutes. Again cat starts to get hot. Thinking upstream o2 sensor or fuel pressure regulator. Everything I read leads me to believe there is too much fuel being burned in catalytic converter. Please help. Have good mechanic skills but diagnosing this is testing my limits.