Denny, your valve cover indicates a leak, as you already suspect. Surface cracks have a way of progressing, so my guess is yours finally reached the water jacket. It sounds like you did a cylinder leakage test on the engine in the past. I would do that again. My limited experience with cast iron cracks is 1) they don't stop; 2) they are hard to repair, but repair is often possible. Good luck with your situation, and let us know what you find.

Harvester