1998 STS is starting to miss, WHY?!

The car has 71,000 miles on it. New Engine replaced at 40,000 miles as well as the Transmission and Air Conditioning. Car was running pretty good, thank goodness for Warrantees. Now, when I start to go up a hill or
accelerate the car starts to miss until I let up on the gas. Could it need a tune up, new plugs, ets.. Some one said it might be the oxygen sensor, if so where would that be on the car?
We like the car but it has been very challenging to say the least. I thought we had everything replaced that needed replaced until now.
TIA
Terry
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going bad among other things. Post here...http://caddymb.netgetgoing.com / for better advice than i can offer.
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