Need engine RPM help on a 2000 3.4

I bought a project, got a 2000 ext cab 6cy 4x4 automatic for $350 and I'm knocking out my list. Here's the problem.

I crank it up and it drives perfect for 20 minutes or so then the RPMs won't go above 2200. I can use the OD button if I need to pass but eventually it doesn't help. It will then stall at a light every now and again. It's got to be a sensor or something thst controls the automatic transmission huh?

Any ideas? I far from a mechanic...

 

No thoughts?

 

Throttle position sensor maybe?

 

Thoughts?
Is the CEL on?
Are there any codes?
Have you checked you catalytic converters?

 

No lights on. I'll be getting it checked soon. Started sputtering on me this afternoon.