I installed a mixer shower over a year ago, and everything worked fine
for about a year. The last few months though it only works around half
the time. Cold water comes through, but when you turn up to hot, it
will run for a few seconds, and then stop. If you turn the bath tap on
and off quickly it will force a quick spurt of hot water out the
shower, but then nothing.

It doesn't follow any pattern, such as time of day etc. When it works
the pressure is excellent. I've taken the shower valve off to see if
water comes through, and hot water runs freely (no apparent blockage).
There just doesn't seem to be enough pressure at times to force the
water up the shower shaft and out of the head.

Info: Mixer shower. Gravity fed (vented) - Cold from tank in loft.
Hot from hot water cylinder (immersion heater). Sufficient clearance
from top of shower to bottom of tank in loft (according to
manafacturers guidelines).

Any further info required let me know. Would appreciate any ideas or
suggestions. I'm moving house, and would like to get this sorted
before selling. Thanks!

Red.