Good suggestions from Tom in order to track down major problem(s).
How does the coolant look? I wonder if it was properly flushed when the new radiator was installed.
Also, make sure the condenser isn't clogged with dandelion seeds, dirt, and debris. No or restricted air flow isn't going to help cooling.
If the thermostat wasn't replaced it may be a good idea to just throw one at it since they are cheap and easy on a '98.
Last on my simple list, but probably would have been noticed, does the fan work?