Is it caused by a sensor malfunction? Where should I look? Help!

it could be a bad sensor in the transmission, however it could be as simple as your idle being to high.....when you put your car in drive if the rpms are too high then the tranny will sense the need to shift and do so.....when it upshifts it then immediately downshifts because the rpms are then too low.....the solution: set your idle slightly lower DISCLAIMER*......this suggestion only works if this is occuring at idle.....if its occuring during normal driving then i would suggest you have it looked at by a knowledgable technician because its more than likely a sensor problem or dirty valves in the valvebody