e34 535i 1990 shifting problem

once a week or so the tranny (automatic) downshifts as if it is being ran with the downshift switch (underneath the gas pedal) is engaged. Shifts at high rpm's and wont go into overdrive. I replaced the switch, no luck still does it every now and then. It is also starting to not shift into reverse. It does this about once a week. Could it be something else besides mechanical problems with the tranny since it is soo spuratic (computer??), or do you think i should start looking for a used tranny