If you are doing the work yourself - by all means take out the heater matrix and back flush it with mains water - and see if this cures your
problem.
Lots of posts on hear regarding heater matrix.
I found the usual garden hose adapters fit will in the matrix allowing good pressure.
I flushed my matrix many times hoping each time would be the last, but it turned out to only be a temporary fix for me - I eventually brought a new matrix and haven't had the issue since. You should flush the whole cooling system first before fitting your new matrix.