Remove the radiator cap and look at the bottom side. There should me a brass disk (about the size of a dime) in the center of the rubber seal. Check if you can EASILY lift an edge of the brass disk with a fingernail. This disk is the valve that lets coolant back in the radiator as it cools.

If there is no brass disk, then you have the wrong radiator cap. If the disk doesn't lift easily a little, then the cap is defective. If the cap is OK, then check the overflow tank and the hose to it for a blockage (as Es 97Sport recommended)

89 Raider V6, manual, 5.29 gears, ARB front locker