Problems it could be:
I would check all the vaccume hoses that run behind the intake. Make sure they arent cracked or anything. Check each plug wire with a timing light, and see if any are all firing. Make sure the wires are plugged into the right spot on the coils. Do a compression check, to see if you have a bad cylinder. Pull the valve cover, and with the engine running watch the rocker arms, to see if they are all moving up and down.
checking the Head:
Check the plugs to see if any have moisture on them, or look to see if any appear cleaner than the others. Usually water passes into a combustion chamber, it usually steam cleans the cylinder, and plug.
Pull of the radiator cap(cold) and see if a bunch of bubbles are coming up to the holw where the cap goes. If you see a bunch of bubbles, this could mean that air is getting past the combustion chamber, into the cooling system.
Good luck.