When you drain it to change the fluid, also clean the filters. When you drop the metal sump plate at the back (6 screws, I think) there is a course wire mesh. With the proper tool, or an easily made-up lever, you can unscrew the filter plug and there is then a very fine filter, which can be removed and cleaned. Check with the book as to which plug to remove.
It can be worth overfilling the box a bit, by tilting the car so that the level plug is raised.
The type of oil can have a big effect - see lots of previous threads on this subject.