My truck (2000 freightliner) is overheating even though the thermostat was recently changed, after working fine for 2 months.. The radiator feels partially cold and warm. The two hoses leaving and going to the radiator are hot and the release knob on top of the top hose when released antifreeze blows out. The resorvoir is full and there is sufficient antifreeze within it. Also, one of the hoses on top is expanding. What could be the problem?

My bet is that you need to replace the radiator, due to mineral buildup or corrosion. The link below may help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563-3.html#post235973

Since you have a diesel, I would change the coolant every two years, with a 50/50 mixture of the recommended antifreeze and distilled water. Every 15,000 miles, add 10 oz. of SCA to prevent cavitation damage to cylinder linings; otherwise, your engine will eventually be suitable for a ship anchor.

Believe it or not your new thermostat could be defective