Once it reaches operating temp reverse slips

I have the vulcan engine which is the regular 12 valve and it is not the flex fuel model. It drives perfect, all forward gear shift strong with no slips at all, engine runs strong as well. Passing gear works, no shutters in 4th gear. When the car is cold reverse works great, once it reaches operating temp reverse slips. It has had the filter and fluid changed but not complete flush, the pan had no metal in it. The fluid removed was not burned, it was just a little dirty. I was told to try lucas no slip so it was added when the fluid was done, it still slips reverse. I know it can't be the clutches or bands because reverse and 1st use the same one and 1st does not slip.