Dear Experts,

I'm visting my parent's house.
The plumbing in their bathtub is really odd.

You turn on the hot water. You get a certain flow and
heat. As usual, I then add the cold water so it's not too hot.

Then, the hot water flow starts to diminish. The water gets colder.

When I turn off the cold water completely, I see just
a trickle of hot water! So, I will open it up some more.

After I then open up the cold water, the same thing will happen!!!

Finally, after two iterations, I have to open the faucet almost full
to
get a consistent flow.

This takes a minute or two to adjust when you are trying to take a
shower.

They have two faucets. One for hot, and one for cold.

What is the cause and the solution? Change the valve?

Thanks a lot!