My power shower works perfectly most of the time, but sometimes
(perhaps 5-10% of the time), usually but not always in the morning, it
begins a cyclical surge, delivering good (normal) pressure for about
one second, followed by what appears to be umpumped pressure for
another second. So about a 2 second cycle, perhaps marginally longer
and very regular. The water during the surge parts is warmer, which I
first thought indicated a hot-water supply issue, but then it's
possible there's a 1 (or 3, 5 etc) second lag from when the pressure
changes to when the temperature change is felt by the user, so perhaps
that is not conclusive. The surging is not instant on-off as I have
described but more like a sinusoidal waveform. There are no other
clues I've noticed; e.g. there is no air noise, there is no additional
or different motor noise when it happens. It doesn't seem to relate
to whether someone else has just had a shower, whether the shower has
been running for a long or short time or outside / inside temperature.
The only image I can picture in my mind is that of an object in the
cold water tank that sometimes drifts over to partially block the
supply to the pump. I had a cursory look (for dead rats, etc.) and
there wasn't anything.

I don't know much about different types of power showers but it
appears to be of the type that pumps both hot and cold supplies. The
pump is an Aqualisa 2.5bar Type 1.5. Looks sort-of like this one:
http://www.aqualisa.co.uk/Our-produc...Pump/?area=202
(that's probably a later model). It was installed by the previous
owner and is at least a few years old so probably out of warrantee.

Appreciate any useful diagnosis tips.