I have a freiend in Oxfordshire who specalises in Auto gearboxes, if there is a problem he will know what the issue is and can probably come up with a solution.
Let me know if you want me to put you in contact with him.
Looking on various forums, it would appear that Mercon V contains a special "friction modifier" which is necessary to allow the special friction clutch material to function properly in our boxes (4R70W or 4R75W). The slippage mentioned could well tie in with that description of what the 'V' does that Dexron or normal Mercon does not?