Where i would start is by doing a compression check on the motor just so you can rule out any problems with that. If that turns out to be okay changing the spark plugs would be next in line. You usually wont see a bad plug by looking at them unless one has a crack in it. If coolant came out the top of the cap that indicates a problem. Might either be a bad cap or a head gasket burned into a coolant passage. Get a block tester and check for that just to be on the safe side.