Good to see that you have found the solution, and thankfully it was a simple and inexpensive fix.

I've had too many aluminium engined cars over the years that often suffered from sludge buildup,either due to poor maintenance or lack of use, that a simple precautionary flush through of the cooling system on acquisition , can help avoid sources of potential overheating problems.

This forum is an excellent resource, maybe it could benefit from a known cures section, which would concentrate the accumulated knowledge into a reference section, so the potential solutions are not lost in the mists of time and among all the background noise.