Yes, sounds like your system may need fushing and treating, have come across it many times, I know it sounds like a lot of money, but if the system is sluged or scaled up, no matter what you spend on parts to fix the boiler, in a number of months you will be in the same situation.
A quick check, if you bleed a ltiile water from the air vent of one of your rads and is is black-ish (sludge) or reddish (rush) your system needs flushing and treating (a good power flush should take between 8 - 10 hours to do (make sure you get your monies worth)
Y
April 2008