BTW - Whenever I change my t-stat, I also pull off the lower radiator hose and one heater hose. I use the garden hose to power flush the heater core, then I stick it in the t-stat hole. I'll run it until clear water comes out. Lastly I'll flush the radiator. The reason I pull the lower hose is because the radiator drain doesn't allow the water to come out fast enough.

 

I also like to pull the lower hose, reason is that any crud that has accumulated is removed by draining from the lowest point of the system. I have found broken hose springs, rust, radiator leak sealer and who knows what junk by pulling the lower hose loose. I would have also ran the water hose through the radiator as well as the heater and T-Stat opening.

How did the radiator core looked like when you had it drained? It should look nice and clean. I have seen the core holes almost completely plugged up. If it looked bad you may want a new radiator.