schizophrenic automatic transmission

I just purchased a 93 Impreza with 175,000 mostly freeway miles.
Usually the automatic transmission shifts smoothly. However,
sometimes when under moderate acceleration the transmission cant seem
to make up its mind what gear it wants to be in. This happens mostly
with the 3rd to 4th shift, but has also happened a couple times in
the 1st to 2nd shift. As soon as the shift to the next gear takes
place, it will immediately shift back down again. When on the freeway
this can continue back and forth till I manually shift to 3rd or give
it much more gas in which case it will stay in 3rd, or back way off ,
in which case it will stay in 4th. Is the shifting controlled by a
computer chip? Is this something an adjustment can fix? Thanks for
your input.

First, try the Cheap way. Change your transmission filter and then, go to a service Center and get your transmission fluid flushed. It is a machine that hooks up to your transmission and pushes out the old fluild and replaces it with flesh clean fluid. Filter will cost about $10.00 to $30.00 and the flush will cost about $69.00 to $120.00.
Try it, it MAY work.