Auto Trans slippage, just after I take off throttled between easy & fast? on 2000 Dodge Ram 1500

From a stop and easing into the throttle I have no slippage, if I apply a moderate amount of throttle like I'm trying to jump into traffic or something it slips after coming out of 1st gear and slips until I let off the gas, it reengages and I go on. Once 40mph is achieved it doesn't slip unless I pull a load behind my truck and I'm going up a steep grade. The trans fluid is clean and no burning is present. I'm confused.

Asked by for the 2000 Dodge Ram 1500