Why does my car sputter when i start it?

I have a 2000 honda civic lx. My radiator blew one day and and had a friend replace it for me. I then had to next replace the muffler. My car ran fine until the muffler had to be replaced. My car then started to sputter while I drove it. i then replaced the muffler and got a tune up. this stopped the sputtering while I was driving. I then started to have coolant problems again. I went to get this checked out and the water pipe gave, so I replaced it along with the timing belt and other belts. I now have sputtering when i start the car for about 10 seconds. It may be worse when the weather is colder rather than nothing when hot, but the weather is to inconsistent here to judge properly. I tried getting some liquid sealant to run through my radiator. when I bought the good stuff it seemed to stop the sputtering but a couple of days later it got drastically colder and went back to sputtering. It seems if I have just gone for a drive long enough to heat my engine that if i stop and turn on and off the engine there is no sputtering. I have checked the dipstick and there is no white in the oil. i have checked the reservoir to the coolant and there is are a few black spots of liquid and the overall color is tinted gray instead of bright green. I feel this is not oil but is the fiberglass sealant that i have added to the radiator. When I look at the exhaust system when it runs it smokes a slight white color but it looks like it is nothing abnormal. Along with this smoke spits out a few water droplets at a time,continuously. When I look at the engine while it runs it doesn't seem to leak anything and there is no fluids splashed anywhere.I am hoping it is not the head gasket. Does anyone have any ideas? I also had a pressure test done and that's when the water pump was fixed. I asked them if they ran the pressure test again and they said they did and everything turned out fine. All work was done with the exception of the radiator at a suburban monro muffler shop. I figured kinda pricey but they seemed less shady about there workmanship than a lot of places. I would assume that all work done to car thus far was done aquatically enough to dismiss reason for the problem.