R reg is a mk2 mondy, if the fluid is as you say brown this indicates the brake bands are breaking up, the boxes are not as bad as everyone says, the early boxes, ie earlier than your one did have many problems, 95% of which were sorted under warranty when new.
Very unlikely that a fluid change will help, a lot of the problems can be cheaply sorted such as the turbine speed sensor, and change switch unit.
Stop wasting your time and go to a trans specialist.
99.9% of failures are caused by excess heat, or neglect, best thing you can do to any auto is to fit a oil air cooler, I've known mondy auto's looked after hit 130-140,000 miles so comparable with a manual, and needn't cost that much more for full recon, obviously dependant on faults, my T5 box has just cost me £580, would have been another £100 or so if he hadn't cut the 5th gear bearing to sort out cheaper.
tabetha