O2 Sensor Troubles

My 2000 Ram was hesitating and sometimes knocking. It was diagnosed as a faulty upstream O2 sensor so I replaced it. The truck ran great for about 2 weeks and now it will hesitate only for 5 seconds or so after the truck is started. I'll back out of the driveway, put it in Drive, give it some gas and it will hesitate and lack power for just a few seconds. Then it clears up and runs like a champ until it sits for 30 minutes or more and I go to start it up. Could the same upstream sensor have gone bad, could the downstream now be bad?? Any ideas?

I have a 1999 Ram, and I was told is would be best to replace both sensors at the same time. I replaced them hoping to solve my drop in gas mileage. Have you had it diagnosed again?

It does seem that one of the senors could be the problem, but my manual states that the sensors send out a flat signal until they reach 600deg. and this is time time you are having issuses...before the sensor reaches its operating temp. I would try replacing the other sensor, and see if it helps. Although, there are ways to test the sensor to determine if they are working properly. I have over 100,000 miles on my truck, so i justified the cost as "preventative maintenance" and replaced both without testing them. Hope this helps.