I'd suggest general "crudding up" of your coolant circuits has occurred.
First get the whole lot flushed - whether by a repair shop or yourself. Vinegar is recommended by some; hydrochloric acid by others (dilute it 1:10 and observe safe work precautions). With vinegar I would add a couple of quarts (after draining the antifreeze and adding plain water less that amount); with acid you could probably add about a pint and a half in the strength sold for cleaning concrete and bricks at hardware stores. Again, you would add it to plain water. Run the motor just a few minutes to mix it up, and let sit overnight.
Next morning, run the motor until the thermostat is 100% open, and make sure the heater is on full also. With the motor shut off, and exercising extreme care, crack the lower radiator hose at the radiator end AFTER it has cooled enough to touch the header tank and you've cracked the cap. Drain the muddy mess into a bucket and rinse off spills immediately; they will corrode your car.
Assuming the block/head is cool enough to touch, get in there with a hose and start rinsing through. Remember residual solution is inside, so watch those splashes! Now start pulling bits off. Disconnect the heater hoses at the carb and at the pump, and flush in each direction. Remove the radiator entirely, flush upside down and right way up. Take off the thermostat housing and blast away inside. You need to complete the rinse before that acidic "mud" has dried again inside the cooling passages.
Test the thermostat and reassemble to its housing and onto the head. Check the radiator cap rubber seal, and the overflow tube or coolant catch bottle. Inspect all rubber hoses, and refit the radiator. If you can afford it, an inline coolant filter may be something to install. Old-timers just slipped some pantihose over the upper radiator fitting before clamping on the hose. It makes a filter of sorts.
Anyhow... That should get you started. Remember to be safe when working with acids; that especially includes your eyes. I always have a 5 gallon bucket of cool clean water next to me in case of emergency (you can pop a hand into it, slosh the acid off your leg, or even tip it over your head).
Cheers, Adam.
