2004 Grand Prix GT, when accekerating normally from a stop it feels like the torque converter locks up at about 2000 to 2200 rpm. Locks up fairly hard. It is intermittent and only happens once every for to six times. Doesnt feel temperature related. Fluid is new with a new filter. Pan and magnet looked normal when changed. Problem before and after change. What controls the TC lock up and how easy it locks? Besides the intermittent problem all other shifting and gear enguagement feels fine. Any ideas? Thanks!