Above 3500 for extended periods of time will heat up the cylinders and cause KR. You may want to back down to one range cooler plug say Autolite 605 gapped around .055"

Any gunk on the underside of the radiator cap? If so flush

Being a 97, assuming the original radiator..you may have a lot of debris in it or the condensor. I'd try cleaning and straightening fins. Especially if you are in a warmer climate. Which ...i'm assuming you are based on heating up this time of year.