I had the exact same problem with a house I rented once. Electric shower was constantly hot, cold, hot, cold. Found out that the mains water supply to the shower was fed by 25mm copper, which is the wrong size.
Simple option could be the shower head. Remove it and give it a good clean
I'll investigate the shower head, it's one with the mineral balls in which I've never used before but thought it was a bit rubbish. I might just buy a new head to rule it out.
I had the exact same problem with a house I rented once. Electric shower was constantly hot, cold, hot, cold. Found out that the mains water supply to the shower was fed by 25mm copper, which is the wrong size.
Simple option could be the shower head. Remove it and give it a good clean