1998 Buick Skylark Engine over heats.

As far as the radiator fan, try unplugging it, use jumpers, run B+ and ground off the battery posts, to see if the fan will run, if it doesn't, you found the problem. You probably should use an inline fuse with the jumper. I have seen bad brushes cause a fan to run intermittently.

Did you check for codes? Some of the national brand autostores will check codes for free. The cts could cause the fan not to come on. You could do some testing at the fan relay. With the relay unplugged, there should be two hot terminals at the plug. The relays usually have a wiring diagram on them, should show the coil side and switch side of the relay, if you use a jumper between the two terminals for the switch side of the relay, the fan should run, use an inline fuse for the jumper.

I wouldn't replace anything, until I was sure of the problem.