Drop in engine power on 2000 Nissan Frontier

Hello. I have a 2000 Frontier, six cylinder. Last month the engine power dropped dramatically after the engine warmed up__to the point that it is barely driveable. Again when the engine is cold it drives perfectly___only after warm up do I have the problem. The check engine light does not come on. Any thoughts?

Asked by for the 2000 Nissan Frontier
This sounds to me like an emissions problem like the o2 sensor or cat. convertors. Are you getting a code?