I found gas valve acting erratically. Shows signs of water on/in valve. I replaced gas valve. Tested operation. Newer vent piping is rusting out due to down-draft of flue. Found basement pressure negative (-.012 inches water column) due to leaks in return-air ducting. This is causing downdraft of flue and fluegas spillage at water heater. Suggestions: seal all leaks in return air ducting or install make up air venting to main level.