when I stop driving, I can hear/see the overflow from radiator bubbling

What is your question?

Car does not overheat, and seems to run fine, but when I stop driving, I can hear and actually see bubbling going back into the overflow reservoir from the radiator. I don't see any leaks? 2000 Mazda Miata with manuel tranny. When I opened the hood, I can hear an intermittent hissing sound (is not constant, and nothing visible). I can also smell anti freeze, but assume that is from the bubbling into reservoir?

What makes this problem better or worse?

Not sure. But of course driving causes the engine to warm, but not overheat

How long has your 2000 Mazda Miata had this problem?

a few weeks?