I actually have an A-999 lockup trans freshly rebuilt. It has the low first gear and new alto clutches. I have had the pan off twice in 300miles and both times noticed black sediment in the bottom of the pan??? Trans fluid looks great (synthetic) with no burning smell. Trans pan is stock.
I did have a problem with slippage and tightened up the low gear band and it hits nice and hard now. While I had the pan off I also tightened up the other band to spec.........

I did install a transgo shift kit per instructions and have no slippage but the 1-2 shift gets weird at full throttle only. At 6,000rpm it made a couple of bangs and the trans was momentarily dis-engaging?? 2-3 shift is fine and since I changed the pan gasket yesterday it seems to not be as bad but it still is not right. Fluid level shows about 1/2 quart on the high side and I am using a nice plate style trans cooler..... Anybody know what this is?? TIA TBF