Central heating radiator doesn't gets hot sometimes and sometimes does?

look like system need good clean up ,add boiler cleaner in header tank ,drain boiler before adding ,follow what on ,run for week ,drain out open radiators valves ,and air vent to flush out ,close all air vent ,refill system ,add restorer ,in header tank , take air out one by one ,and balance system ,close half the one hotter ,full open which are not

ltd
December 2013