Running Lean with poor throttle response

I'm very new here and I just got my 2000 GT a little over a month ago. The check engine light came on and I got the P0171 code. I changed the spark plugs and oxygen sensors but the check engine light still said it was running lean.

Here's what it does:
When you first start it, it will idle rough for about 3 to 5 seconds. Then it will accelerate itself and idles fine. It only does this when you start it for the first time in several hours. If I start it, shut it down, then start it again, it will not idle rough the second time.

The other problem is that it has not great throttle response. It isn't terrible but it down take a second or 2 for it to accelerate like it should.

I have changed the oxygen sensors, the spark plugs (but not the plug wires or coil packs) and the fuel filter.

The car runs well with virtually no problems. But I want it to run perfectly. How can I get rid of that poor throttle response so it will accelerate right away?

All help is appreciated. Thank you.