This is an important question, if it'll sit and idle and seem fine, but driving makes things worse this might point to blockage in the radiator. The "Garden hose flush" is good as a preventative measure but won't break up scale wedged in cores. On a warm engine the core temp tells the tale, much cooler at the bottom indicates a clog. Normally the coolant is cooler at the bottom of the rad, but it's still well above 130* before re-entering the engine. If your '89 has A/C and the shroud/clutch fan, access to the rad core is limited but you could shut off the engine and QUICKLY feel around the core by hand for cool areas.
How is the resistance of the fan's clutch when warm? It's normal for coolant to enter the expansion tank whilst engine reaches normal op temp, and to be slurped back in as it cools. Half the rad's contents sounds extreme, how much did you actually add?
Another simple test for combustion leakage is to fill the radiator completely up to the overflow tube (this can take a while as coolant will settle, especially if the bleeder is used). With coolant level stabilized, remove harness wire from ign coil and crank engine while observing open rad filler for bubbles (or giesers!). Should have a lab assistant in a wetsuit and goggles for that test.
Another test for radiator flow; Remove both upper and lower hoses and allow rad to drain completely. Next "fill" the radiator with briskly running water. If the flow of water is greater exiting the upper hose connection you've got clogging in the core. From a full force garden hose almost ALL water should exit lower hose connection.
There's more, but try these simple (and free) tests first.