My downstairs bathroom sits over a utility room in the basement, and you
can see the pipes that go to and from the toilet in that room. A few
months ago, I noticed water on the floor and thought I had a leak. So,
I had my roommate go to the bathroom and flush. Here's what happened:

Toilet flush.... everything OK
Tank starts filling.... starts leaking into the utiltiy room like crazy
Tank completes filling.... leaking totally stops

I found it strange that the leak only occurred when the toiled tank was
filling. Once it was full all leaking stopped. Since it did not leak
during the flush, I assume that the water was clean. It seemed strange
and didn't know how to diagnose it.

Any ideas?

It gets even more complicated:

So, I made that bathroom off limits until I could find a plumber (which
I never did), but my roommate accidentally flushed the toliet there
today. Since I was downstairs, I ran to check out the leaking. Well,
guess what? No more leak. Now I'm really confused, and how am I going
to ask a plumber to fix a leak that no longer exists?

I am asusuming that the leak stop is only temporary, either that or the
plumbing gods have smiled on me. Any thoughts, ideas, or opinions are
greatly appreciated!

Thanks,

Mike