[slideways...]

its probably because your tranny is dying
flush your tranny fluid and change filter. it might help alittle if your fluid is old
just having enough isnt ok if its old fluid
the oil tends to break down over time and with the constant gear motion that it lubricates its like putting it thru a blender.
oh and i would assume that you know to check the tranny fluid while the car is running?
let the car warm up and shift a few times between P, D, R, N, and check the fluid after that.

if this doesnt help then its time for a new transmission