Over the past month my 89 240 has had trouble staying running. I the car starts great but it won't stay running. We thought it was the air flow meter which turned out to be fine. Then we changed the O2 sensor which seemed to work for about a week and then the problem came back. I thought maybe their was a bad connection on the wiring as we installed an after market sensor. So I soidered the three wires and put shrink tube around them but the problem still remains. Is their something else that I could have missed in another area or maybe we got a bad 02 sensor? Or, when you use the after market 02 sensor rather than the factory ones these are typically what happens? I just could not see spending $250 for a factory one when an after market one was more than half the price. I'm looking for some ideas or suggestions for what to try or do next????

Thanks for your help in advance,