If you have access to a tempurature sensor such as used in the plumbing trade or even a domestic cookery thermometer, take some temperatures from different locations across the cooling system: radiator top and bottom, top and bottom hoses and different parts of the block and cylinder head.
Tempurature gauges in cars, particularly old ones are notoriously inaccurate. A calibrated sensor should help you identify whether you have a problem and potentially where.
