Most likely, either the thermostat is sticking, or the radiator is really clogged. Check if there are any deposits inside. It also happens that the pump is pumping weaker. If you want quality spare parts, I recommend HD ProParts . They have a huge selection of radiators, thermostats and pumps for trucks. The main thing is that they sell proven parts - I myself bought a radiator for Kenworth from them, the difference is noticeable. You can quickly select spare parts by model, delivery all over America. Don't delay this, overheating is a dangerous thing and can lead to engine repairs.