What is causing the misfiring under load at higher RPMs?

My truck is miss-firing at about 4500 to 5000 RPMs. It usually does it when I go into passing gear. Most of the miss-fires are in 2 but also in 5, 6, 8. I have changed the mass air flow sensor, O2 sensors in bank one, spark plugs and wires, and cleaned the throttle body but I am still having the same problem. The truck runs great at normal driving with no miss fires. I am also having trouble trying to perk the coolant system. I recently replaced the coolant flange on the right side of the motor because it was cracked and leaking. After I replaced the part and put everything back together, I attempted to perk the coolant system. I put the heat on high and filled the reservoir and waited for the thermostat to kick on. I sat in the car and after about 5 minutes I could feel the hot air coming out of the vents, but moments after that cold air began flowing and the car would just continue to overheat.