Mira shower problems?

It may be because of cold water - the shower stalls until the desired temperature is reached. Good to begin with because the temperature of the water entering will be that of the house - then as the flow continues much colder water enters the shower and needs more power to warm it up.

Your plumber(s) may neglect to explain this - they get much more money if they install a new shower for you.

Does this problem happen only in the winter?

Your statement - There is no difference if I am using other water appliances - is confusing.

I know nothing of showers - I just guess. I think the shower(s) are limiting the flow rate due to the difference between the temperature of water coming in and the temperature required for the heated water going out.

Stay clean...
January 2016

"after a few seconds all the lights flash"

All meaning what, exactly?

There are, according to the manual I found at http://www.mirashowers.co.uk/onlinecatalog/pdf/1073791_w2_b.pdf FOUR lights but pages 23, 24 and 25 of that document suggests there is NO fault indicated by all FOUR lights flashing.

Please clarify or RTFM a bit closer.

XXX
January 2016