Japscrap_96

I own a 96 240sx manual with 193k miles on it. I'm having a bery specific problem with it. When I first start the car it drives awesome for the first 30-50 mins, after that it begins to lose power. For example, if I come to red light and want to take off the car barely moves (no pull) but the RPM goes up. This same thing happens on second and third. It takes really long to catch speed. Even if I floor it!
It's alsmost as if the clutch was slipping. Two weeks a go a technician friend of mine replaced the clutch for me, but the problem didn't go away. We scanned the car and got 5 codes (p0304, p1447, p0446, p0440, and p0325) The most important ones where misfire on cylinder 4 and knock sensor. So last night I went ahead and did a tune up to the car. I bought NGK plugs and wires, new cap and rotor, and bought a knock sensor. The misfire went away, the cap was cracked. I drove the car that night for an hour and 10 mins (the longest its lasted without losing 50% power or so) I called my buddy and he said it could be an O2 sensor or a coolant temp sensor. So I bought those two and put them in, I also went from mid grade to premium gas. The car drove like a dream. I even did some skids. An hour into the fun drive the car lost power and was barely moving. Now I'm here sitting trying to figure out what's wrong with the car. I would really appreciate it if you could help me out. Thank you so much.