cracks in wood flooring

I have a home with 170 year old pine flooring. We love it's character, but some areas have large cracks in the floor boards, as large as 1 to 1.5 inch gaps. About 20 years ago, a previous owner tried caulking these areas. Not only does it create an obvious difference, but now it is cracking and falling out in pieces. I have tried experiments with sawdust and tinted wood glue. The color was good, but the wood absorbed much of the glue and left a rough surface. Has anyone had better luck with a different combination?