Bathroom boiler problem?

hi, i have a little problem with the sink and boiler , its a 2 in 1 tap so its both hot and cold, when i use it i turn the hot one on, and its so hot i cant even touch the water, so i go to turn the cold one, if i turn it like more than a quarter way the boiler switches off, and the waters coolin dwn from the tap and thats when its touchable. and ideas what could be wrong? by the way this doesnt happen in the bath, only the sink (i hope that made sense)