Why does my car's engine lose power on accelerating?

What is your question?

It started out last year by simply just not wanting to up-shift. It would act like the spark plugs/wiring needed to be replaced, but the spark plugs are new. Slowly, it got worse and worse and worse, then finally it started to just lose power instead of being stubborn and sluggish shifting. Particularly when I am accelerating, I'll get through first gear and it will be fine most times, but then after shifting into second, it will lose power and be almost impossible to accelerate. It feels like I have let my foot up off the gas pedal when I have not. Gas mileage has been continually getting worse. I have noticed that the engine will work fine until it gets heated up a bit. And then once it's about a fourth way heated up, it will start acting up. Once it is half way (or normal temperature for a engine), it will usually regain some or all the power. But it does this every single time the engine gets warmed up to a certain temperature. I have replaced my fuel pump and filter. I've replaced my Mass Airflow Sensor (or MAF), Manifold Absolute Pressure (MAP) sensor. I get oil changes regularly. There is nothing leaking from the car. I had a new EGR valve put in, throttle position sensor. I believe that vacuum hoses are often controlled by temperature. Could that possibly be the reason since it only does it after hitting a certain temperature? And if not, what else could be the causes?