Power shower surging

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:

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(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.

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James Hawkins
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