Car normally over heats. Possible that previous owner put additive in radiator?

yes bars stop leak has black pellets in it. they melt and tend to do this goo thing.your suppose to remove thermostat when you put it in, you could have plugged sections of your radiator, at least flush the system you can get a connector to hook up a garden hose at local parts stores..... but id replace your radiator SOOON because this is the only component that effects the transmission in cooling system.. smell your tranny fluid to see if you smell a burn smell if so do a fluid change quickly head gasket failure and transmission failure will be sure to come if you keep driving it like this