00’ Sonoma loss of heat, temp rising

[bdavis4623]

first heat went out, but would work at Highway speeds. then temp would start to fluctuate, and also noticed a leak at passenger floorboard. from what I was told all signs pointed to heater core. replaced that, still same issues. oil looks fine coolant looks fine aside from quite a bit of corrosion and gunk caked at the top of radiator, and below oil cap looks pretty messy as well but not as bad… it doesn’t quite look like oil mixed with water yet, hoping just very old coolant needs flushed, what say you??