Intermittent coolant loss & overheating. Internal leak intake man. gskt. anyone?

Forgot to mention. Combustion leak test was done in shop, showed negative at time test was done. Oil not "milky" like it gets from coolant and oil swapping holes in head gasket. Am familiar with that. This is why we don't suspect head gasket yet. Thought of pulling spark plugs and looking for signs of one plug cleaner than others maybe meaning coolant leaking into that cylinder from somewhere. It's a stinking intermittent problem. Right now running o.k., and not overheating, but expect it to rear its ugly head soon. Not sure if there is a coolant passage running through intake manifold gasket - trying to find out. Have seen it in GM V6 - replaced with better-designed gasket, solved.