904 Slippage

I had the same thing happen to me before on my trucks first transmission. In my case the trans had a lot of miles and the fluid hadn't been changed regularily. The clutch material in the fluid kept the clutches engaged good enough, but when the fluid was drained, so went the clutch engagement. How many miles does your trans have? If it fits the above scenario, you might be looking at a rebuild. If you kept the fluid changed and the trans is relatively low miles, then maybe the band adjustment was done wrong.