It should almost never hit 100°C on the gauge. Make sure the radiator fins are clean, and also the AC condenser fins. Clean with compressed air if possible. Make sure the radiator isn't cool directly in front of the fan clutch, or the clutch will never engage (even if brand new). The clutch is suspect, these are known to be problematic... to test, get the engine to 100°C or higher, pop the hood, and kill the engine with the "stop" lever on the IP. Watch the fan - it should stop within 1 turn, it shouldn't freewheel to a stop. Don't forget to check the temp ahead of the clutch before condemning it... a bad radiator can make a new clutch not engage.
After all else fails, a new radiator is sadly quite common... they seem to end up corroding internally, or getting buildup inside the tubes, although they look perfect from the outside and when peeking into the plastic necks. My '87 is on it's third Behr and it looks like it will be getting #4 before next summer. Note that the
factory procedure for curing temps above 110°C specifices a citric acid flush followed by radiator replacement... the acid flush won't "fix" a radiator with excess deposits.
