Intermittent problem?????

When I turned the shower on today it ran for about 5 seconds then stopped. I tried to get it to work by pressing the on/off button many times but still nothing. Then I turned the dial from the hottest heat setting (2 red lines) down to the next one (1 red line) & the shower worked so I immediately turned it back to the 2 red lines setting & it turned itself off. So I started the shower again on the 1 red line setting & let it run for 30-40 seconds then turned it to the 2 red lines setting & it stayed on. Some 12 hours later I went to use the shower again & the same problem happened, so I went through the steps that I had taken this morning & all good. So whilst I was in the shower I kept turning it on & off on the 2 red line setting & it worked fine. Then I turned the shower off & left it for 10-15mins & tried to turn it on in the 2 red line setting & it worked fine. It seems that when I leave the shower for a long period of time the only way to get it going is to start it on the 1 red line setting & running for 30-40 secs move to the 2 red line setting. I’m fresh out of ideas as to what’s happening. The shower is roughly 3-4 yrs old