97' GV 3.3 cooling fan problem

Hello:
i am having a problem with my 97' GV 3.3 cooling fans,problem is that
while i'm in stop and go traffic as you may know engine gets hotter and
the cooling fans come on as does mine,except that mine will stay on at
the higher speed and will NOT shut off at times until i turn off the
engine ans restart it again and they will come back on when the temp
reaches a higher lever and then again will not shut off until i turn
off the engine.
i have replace the cooling fan relay which is up under the left side
wheel well.
thr coolant temp sensor is working properly.
so can anyone tell me what may be causing this problem.

Thank You
Earl