my truck keeps wanting to overheat on me. it doesn't leak water and the reserve is always full. Just changed the thermostat and that didn't seem to work. any ideas on what it could be? i appreciate any help.

 

Could be a clogged radiator. Remove the radiator cap... what color is the coolant? Should be clear orange.

A scan tool will tell what coolant temp is. Feel the upper radiator hose, and both heater hoses where they enter & exit the firewall, all 3 should be very close to the same temperature.