1993 Buick Lesabre Transmission shifting wierd.

I just bought this car a few weeks back, and the transmission is leaking, but not bad at all. I am able to drive a good 250 miles before having to add fluid. The problem is. It is not wanting to shift into second gear, but eventually does shift. Then when at highway speeds it will randomly downshift a gear, making the RPMs ride high to keep up, but the problem is on and off, I have driven on the highway about 80 miles straight at 70MPH without having a single problem, but then on some days I have issues keeping it at 50. Im wondering what to do? I am thinking about changing the filter and gasket out, but I dont want to do it until I know what the problem is. The car is parked for now. Thanks, Jeremy