what is reason/solution for 2004 f250 (gas) stalling in stop and go city traffic

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after 1st occurrence, had regular service and replaced air filter. Problem recurred next time truck was in slow/standstill city traffic. To mechanic again,diagnostic computer identified a problem with speed sensor in the transmission. Sensor replaced and trans fluid changed. (approx. $400)and problem has resurfaced in less than 1 week - all occurrences in slow/standstill city traffic (approx. 150,000 kms)