We had the same problem after talking to the man in our local harware store he suggested that we change steam valve which fixed the problem

http://www.kighthomecenter.com/build_tips/buildtips/faucets/faucet.html

thank you for your response. what exactly is the steam value? i read your link and don't really know which part needs replacing. is it the whole bathtub faucet that needs replacing?

the part under the knob is called steam valve the washer part is what needs to be replaced

He is talking about the FAUCET STEM.

There are literally 100'S of different brands and styles.

Some use a different one for hot and cold.

http://www.faucetstems.com/index.php?cPath=22&osCsid=87af27d4831d33b0a79966204b50a515

A good hardware store or plumbing suply store will be the best place to start. They will often have the ones that are common in your area.

sorry in Texas we call'em steam valves

so i need to replace the faucet stem? i assume that i can take the faucet stem out without having to replace the whole faucet?

the link that was sent to you is for the 2 handle removal only. Should of ask if you had one knob (that controls both hot and cold water) or 2 knob that controls water seperatly if it is a 2 knob you would only need to remove the handle and the valve behind the handle to to replace any worn parts

If you know the manfacture I would also suggest using search engines for repair tips and even going to local hardware store for repair book (doesnt hurt to have one handy)