water temp going up and down?

the problem is that when i take a shower, water stays hot (as i like), then starts to go cold'ish, then back to hot'ish. if i turn tap to make it hotter, it goes hotter, then cold'ish again. and it continues while i take a shower, not just for a moment. left water running, went downstairs to look at boiler. the display was showing: yellow light on,reaches about 65 degrees, yellow off, goes down to 60, yellow back on, goes up to 65, yellow off and so on.
water temp turned about 30 degrees on boiler.
i noticed that it started when hot weather hit UK (it was about 35 degrees). there was no problem in winter or spring.
Vaillant boiler.
it is reaaallllly annoying. any ideas ?
many thx