Some ideas in order of likelihood:

1) bad radiator cap
2) bad fan clutch
3) bad water pump
4) plugged water passages in the lower intake manifold
5) plugged water passages in the heads
6) hardened sediment in the cylinder block water jackets

Since you didn't say you replaced it, my money's on #1. That's the only one I can think of that would allow coolant to boil over without showing an overheat on the gauge - but then, the gauge or gauge sender could be lying.

Frank

'89 [color:"white"]G-Raider[color:"white"] [color:"black"]Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 172k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, Lifeline AGM, Rust-Oleum