code p1870 come's on 60 to 65 mph

Hi There;I have a 96 Grand-Am,3.1 v6. Car shift's fine,don't notice any slipping or harsh shifting at all..just the check engine light comes on after I go steady 60 to 65 mph,like on trip say 20 miles or so will come on.and stay on,but drives ok,even with it on.set's p1870 code. trans shifts ok even with light on.1st through 4th all smooth,even tcc seem's to go in and out smooth........seem's as though the code set at a steady 65mph and when it came out of tcc,noticed the light come on at that point.Could it just be the tcc solenoid itself???.................Don't get any harsh shifting at all, all gears are there and smooth shifting. only happens when crusing 65mph or more steady.