1st step would be to swop out the old fluid with some genuine stuff and add 65ml of DC Moly, all of which I have in stock.
Failing that if the box gets worse then a low mileage preowned box would be the most cost effective option, again boxes in stock, all with 12 months unlimited mileage warranty.
3rd option is an overhaul, which will give you a reconditioned transmission, this can be done on exchange basis or I can recondition your existing box. again 12 month warranty applies.
Alex.