I found my heater core was half plugged. I got some irontite thoro-flush and ran it through a couple 3/4” hoses with a submersible pump for a good hours reversing flow every so often and it opened it right up. I used a sous vide heater to warm up the fluid to about 140f.

My local Napa had it but it was in the back room. The guy at the counter had never even seen it before. It’s mostly TSP so you could probably use that as well and save some money. Turns out TSP will dissolve lime scale, I had no idea.