Besides that, stability is always relative, in the longer term no country is ever stable, in the shorter term in the case of countries like Afghanistan they are stable about as long as other nations don't cross their borders.

Vietnam, Afghanistan, Iraq, Iran and many others besides. All of these were at some point reasonably stable, and then someone somewhere decided to attempt to enlist them in one scheme or another and/or tried to wage a proxy war through them or actively tried to install regimes more friendly to foreign interests.

The Roman Empire in the long run also wasn't stable, but it fell to rot from within, as most countries eventually do. But in the case of Afghanistan the destructive force was applied from without.