2001 Nissan Altima • 160,000 miles

Ok, so I've done tons of research but I'd still like to ask, we haves 2001 Altima with 160K on her. Car starts runs and drives fine, after it gets to normal temperature( thermostat needle at midway) it will gradually rise getting hotter an hotter, and then the temp/needle will slowly go back down, driving seems to make this fluctuate more, and the heater blows cold while running hot. Also after this temperature ordeal once I stop an turn the car off the electric fans blow fine to cool the radiator but then I come back and all the antifreeze(50-50 mix) had either overflowed or leaked out of both the radiator and the backup or overflow tank. I had my mechanic bleed the air out of the system and refill the Coolant. Same problem occurred so we changed the thermostat(and rechecked position) and the same problem occurs. Any help or ideas?
January 25, 2013.

Try replacing the radiator cap.

Jan 27, 2013.