The matrix works by convection, heat from the input travels along and warms the water already present as opposed to hot flowing through, the end tank of the matrix is a open chamber so if the internal pipes are silted up water will just flow in and straight out again, you could try flushing, try Wynns cooling system flush leave in for around 200 miles not the 10 mins on the drive, this will remove all the sludge from the whole cooling system/engine and deposit everything as a congealed mass in the expansion tank which will need clearing out then, but it works perfectly.
be warned though it will also find any leak that may have been sealed in the past with sealers.
Personally though I'd just sling a new one in, do it easy in a good morning/afternoon, all the modern ones are aluminium, most from poland, good quality but no where near as good heat wise as original copper ones.
Why fit a 82C stat if wanting a better heater, it doesn't protect the engine any better or safeguard the HG at all.
tabetha