comments only visible on first page load 6.x

I have comments enabled and when I login as a user other than admin, the comments on a page show just fine. But when I try to change the settings (threaded, etc), or go to another page and come back, or try to reload the page, the comments and the ability to enter comments disappear completely.

I hope someone can help. I've checked with a standard theme (Garland, I am using Light Fantastic) and the problem is still there.

TIA

Terry
OSU