The temp gauge is reasonably accurate with the correct sending unit. This has been documented ad nauseum in these forums. As Larry mentioned, your problem may have nothing to do with coolant temperature, rather, it may be that some other component is close to failure but works when it's cool. The first thing that comes to mind is the coil or perhaps the fuel pump. Next time it happens, pull a plug wire and check for spark. If that's OK, check for fuel.
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