It sounds like the filter may be plugged up. If you use the search feature you should be able to find several threads on this problem as the subject has came up numorous times. Often you can change the trans filter and it may be ok for a few days or weeks and the problem returns because there is a lot of debris from something damaged in the trans that will plug up the filter. When the car is not running the debris will settle down and as soon as you start the car and the pump is spinning in the trans the debris gets sucked into the dacron filter material and restricts flow to the point where it can no longer make pressure. The easiest thing to start with is to drop the pan and remove the filter and cut it open to inspect it.
