My husband's Toyota truck is an 88,4x4 with a standard /stick shift transmission. After 20 minutes of driving, the heat guage goes toward the hot range, but after 10 more minutes, the gauge goes back down. The hose closest to the thermostat is hot, but the hose under the radiator stays cold. What can this be? My husband asks if it's a water pump problem, but I think it's something else.

 

Is the heater set on full hot when this happens. If so, have him turn it to full cold and see if it still does it. If it only does it with the heat on I know what it is.