I have a similar problem with my 99XK8. I first suspected one of the 8 solenoid valves. Mine hesitates before engaging from a full stop, but not all the time. If I gradually press the accelerator it has less of a tendency to pause and jerk into gear. From what I've read the ZF 5HP24 transmissions were known to have problems with the A Drum/Clutch chipping and or burning. There is a improved version available that isn't expensive. It's the labor cost of removing the transmission and replacing the A Drum and putting it all back together. I got estimates of $2800 USD to repair it. My next choice is to get a used tranny and swap it out. I found a used one for $500 USD and a private shop that will swap it out for $675 USD. I haven't changed the fluid and filter yet but will start there and look for metal on the magnets. There is always a chance it's just low on fluid, but not getting my hopes up. It shifts very smooth if it weren't for the annoying occasional jerk from a standing stop. I hope this helps you make your decision.
