Since it is gradually getting worse than you are probably best off getting a rebuild or replacing it, but if you are replacing it with a used one you must make sure it is compatible with your car as there are a number of different electrical and gear ratio combinations available amongst the years. Do you have access to a scan tool to check out a few things or would you have to take it in to a shop to get checked out? A failing pressure control solenoid will definately cause what you have going on but not typically with some other side effects but the only way to see what is going on is with a trans pressure gauge and a scan tool. Also if your maf sensor is dirty from old age or if you have a K&N style oiled air filter that can cause the maf sensor to prematurely get dirty and this can directly effect shift pressure and will be most evident at WOT.
