check the water pump? Sucks to find one for an SR, which is reason 3893823 I won't get one, but it sounds a lot like water pump. My KA was doing a similiar thing when my water pump was taking/had taken a crap (getting up to hot at stoplights, when stopped, etc, then cooling off as I drove). I ended up blowing a heater hose at the rear of the engine as well.

But my first instinct says water pump.

http://www.frsport.com/Nissan-OEM-S1...ly_p_2218.html

Pretty good price on one.