Car problems...Please give advice...metal clanking?

The story starts 3 weeks back when I was driving down the highway and I saw that my car was overheated, I heard a funny noise every time I put my foot on the gas. I pulled over and let the car cool down, added distilled water, and waited. I then tried starting the car, which seemed to be an impossible feat. I finally got it started and it felt like it was about to die. I got the car towed to the mechanic and they told me that I blew a head gasket *only. I got that replaced, timing belt, water pump, and the thermostat. The car seemed less powerful afterwards, and quite a bit louder when I accelerated *I thought no big deal. Anyhow, two days ago I noticed a subtle metal clanking sound when I would give the car gas, it seemed difficult to start, and I would drop RPMs when I would put the car in gear. I thought plugs, I took the car to the parts store and they tested the computer b/c the engine light was on. It said 4th cylinder misfire. I changed plugs and wires. NO CHANGE!!! Please help.