Coolant Loss !!!!

[joe k]

I have a 2003 H2 that has a coolant loss problem. Started to use a small amount of coolant at around 24k miles. Problem has escalated to a loss of about 1/2 gallon every 1500 miles. Have taken the H2 to the dealership several times to address this as a warranty concern, no external leaks have been found. The techs at the dealership have inspected system for gases in the coolant system (dye test / black light), along with pressure testing, ect. It also looks like the rest of the component fluids are clean (engine oil, transmission, ect). Have had 2 dealerships and 2 outside shops look at this problem and no one has a definitive response other than " the coolant is going somewhere." Also had a engine light come on at around 36k miles, turned out to have most of the ceramic insulation missing off of the #2 spark plug (rep at the dealership stated that I needed a fuel treatment before I demanded a plug inspection), have a question in the back of my mind if the 2 problems may be related.
Anyone out there have any thoughts on this? Willing to listen to any thing at this point.