Ok, What have I done now... (904 problem?)

ATF+4 is not a synthetic. It is a friction modified fluid that has the proper friction modifiers for the newer valve bodies. I'm not saying it won't work in an older transmission, but you have an issue with shifting and are using a fluid never intended for that transmission. Just covering bases here. If your clutches are slipping before engaging, it seems to me the fluid could be at fault. And draining the pan after 9 liters have been flowed through the converter, cooler and radiator probably wouldn't be enough. Just my 2 cents worth.

I use Amsoil synthetic in virtually everything I own. It actually fits many applications and backdates to work in older vehicles. It ain't cheap, but neither is a motor or a transmission. I just dumped a 2 1/2 gallon jug of Amsoil ATF into my new 200R4. That one stung the old wallet a bit!