I think that you need to dissassemble the valve to investigate the problem,
which is probably due to loose, worn parts.  What you probably have is a
plastic valve (o-ring seals) that seats itself with water pressure. The water
pressure becomes unbalanced when the hot water heater kicks in and increases
the pressure on the "hot" pipes.  Consequently, the valve becomes unbalanced
and re-adjusts itself to shut down the cold.  The problem can probably be
fixed with either new seals or a new insert.  Good Luck!

Steve Stokowski
Stone Products Consultants
http://members.aol.com/crushstone/petro.htm


In a message dated 98-09-27 23:57:40 EDT, you write:

<< At some point, anywhere from 30 seconds to several minutes from when the
 water is turned on, a "thump" is heard from inside the wall at the head of
 the shower.  The sound is not very loud, but it reverberates a little.
 Simultaneously, the cold water pressure drops dramatically, and the
 remaining water coming through the shower head is, by default, very hot.
  >>