If an O2 sensor gets “lazy” because of old age or contamination, the computer may not be able to adjust the fuel mixture quickly enough as the engine’s operating conditions change.

Intermittent driveability problems of any kind are always a challenge to diagnose. When a customer brings you a vehicle and complains of a stalling problem, you may not have much to go on other than their description of what’s happening and when. The more information you can get out of them the better, because you’ll likely need as many details as possible to narrow down the list of potential causes.