I've got almost all hardwood and boards throughout the house.

In two rooms the hardwood is separating up to just under 1/8". Aside from replacing the entire floor, has anyone come up with a solution to filling the gaps?

Is it seasonal? As in do the gaps appear in the winter and shrink in the summer? There is really no great way to take care of gaps once the floor is installed.

If you refinish the floors you can putty them with a stainable wood filler. But honestly, due to the seasonal shrinking and swelling it will mostly all eventually pop out again.

No, they've been there since I've moved in.

I'm betting they never let it acclimatize for 72+ hours before installing.

I was thinking two options.

Strips of wood with high quality wood glue, then resurface and stain.
And for the small gaps wood filler.

or

Flexible grout. Thoughts?

If they have always been there, then yes, I would agree that the wood was never acclimatized prior to installation.

It would be a lot of work, but the strip of wood would be the most resilient to install.

I'm not sure you could find a flexible grout with a close enough color match to make it look good. Some wood fillers are billed as "flexible" but they will all eventually dry out and crack.

Got any pics of the problem?

After looking at these, I should clean again.

Ha. Cleaning aside, yes, looks to me like an original install problem. I don't know that you are going to have much luck with any repair.

Beat the crap out of it with a hunk of chain and call it a "distressed" floor.