1999 Mercury Villager Villager Mystery

Engine Performance problem
1999 Mercury Villager 6 cyl Front Wheel Drive Automatic

1999 Villager with check engine light indicating 02 sensor bank 1 sensor 2 cat converter. Sensor replaced. 2 days later banks 1 02 HEAT sensor light came on. Drove for about a week with light on. In the same week I drove the car down to empty, pretty much on fumes. Got gas and about 15 miles later it began to sputter occasionally. I thought maybe I has bad gas or clogged fuel filter. Sputtering continued off and on intermittently for about 4 days. Car stopped and when I opened the hood the exhaust manifold with the front converter was glowing orange. Replace 02 heat sensor and nothing happened. Mechanic took off both converters and the front had busted up chunks inside clogging it completely up. He took all the chunks out and put the converters back on. The car continued to run the same. Checked fuel pump it turned out to be alright. The problem still continues. The engine runs smooth for about 2 minutes and the sputtering gradually starts. If you drive it, it will drive and sputter and when you come to a stop it will stall. Spark to distributor is fine, no apparent vacuum line leaks. Code reading still says 02 heat sensor after code is cleared and the vehicle continues to sputter. I don't know what to look for. Any suggestions? Thanks.