A slow coolant leak can be sneaky. My/my daughter's Neon was loosing coolant and I couldn't find it. Had to top it up every week for several months. Then as she was pulling into the garage one day, the radiator just let go. There was a small leak in the corner of the core hidden by the shroud. It evaporated as is leaked so it was not dripping and I couldn't see the white telltale residue from it evaporating. New radiator fixed it.

In your case, suggest you get it good and hot running, shut down and inspect carefully. Also, may be worth trying another cap... I know you replaced it but it sure sounds like the cap may not be doing its job.

If coolant is getting into a combustion chamber, it will tend to steam clean the plug. It would be worth pulling and inspecting the plugs... if one looks really clean good chance it is a leaking head gasket or crack.