My sinks in the kitchen seem to regularly get plugged up, and the toilets will act as if they've just had enough-and overflow! Not every time, of course, just when you think it's safe.
It's a really annoying problem, whatever could be going on?

I read in another thread something about roof vents, and I feel rather stupid, but how is that connected to the bathroom plumbing..
OK! Thanks so much! :)

Since the sink in the paullips on separate lines. You have two separate problems. Let's address them one at a time. First a little more information. How old is your home? Does your sink have a garbage disposal? When you sink clogs up. How do you unplug it?
After we get your sink makes we will move on to the toilet. Back to you, Tom