VIPLexy650

hello everyobody. i was wondering if anybody has had this problem before. i have a 92 ls400 and my tranny sometimes works like a charm and some other times it laggs pretty badd. i would think its going badd but if it was how come sometimes it will work so properly weeks at a time. is there a sensor or something somewhere that might help. also on my dashboard my o/d light flickers on and of all the time. thanks ahead.