Any white smoke or sweet smell out of the tailpipe?
Bubbling in the coolant indicates a possible head gasket leak, try running the engine with the radiator cap off or look the overflow tank if you have one.
Do you have any smell of coolant in the crankcase oil?
How old is the coolant? Coolant loses its corrosion qualities over time. Distilled water used? A flush and refill can’t hurt. I’d take the radiator to a shop for cleaning instead of an off the shelf chemical flush.
I’d do a cylinder compression check and pressure check of the cooling system, the chain car parts pl