My car was overheating a while back and after noticing some grime in the coolant reservoir I suspected the tap water I used in a jam (I know, I know) was the cause. So I pulled the T-stat and put in some coolant flush. I ran that way for a few weeks and it ran fine (although a bit on the cool side)

Yesterday I put in a brand new T-stat and coolant/ distilled water. I burped the system and went for a test drive: still overheating.

So having a thermostat in the car is causing it to overheat. The water pump is fairly new and the impeller felt fine. I don't think it's a headgasket, no white smoke, no coolant bubbles, no oil in coolant or vice versa. I'm leaning towards a partially clogged radiator. Water flows through it but is there a way to tell if it's partially clogged? And what's a good way to unclog it? The coolant flush didn't seem to work.