- Plugged cats
- Low ratio of water vs. anti-freeze leading to the center of the radiator freezing solid at speed in cold weather
- Debris blocking radiator
- Radiator side shields MIA so that incoming air is bypassing
- Missing lower air dam - removes negative zone at rear of engine, defeating proper hot air evacuation under car at speed
- Belt slip on water pump pulley
- Worn out vanes on water pump
- Water pump seal/bearings worn out, allowing air to be sucked in past seal when large negative pressure built up inside pump at speed
- Loose/corroded connector on water temp sender causing erratic readings
- Bad radiator cap
- Leaking heater core
- cracked head/failing head gasket
- ...any combo of above
Be sure that the overflow tube isn't blocked into the overflow tank. Check if the upper radiator hose gets extremely tight when running and/or collapses/flattens when cold overnight.