It sounds like the nut is not secured to the cross slide, or has damaged threads. Could it be that you are not removing all of the backlash by cranking out then cranking back in? I would take it apart and take a close look at the lead screw and the nut as well as the attaching parts.
I had similar problems , that's exactly how I've finally stopped my cutting tool grabbing .
Early on in the problem solving I also removed , washed and oiled all the slide adjusting gibs for the bed carriage , cross feed and the top feed slides then reset them to take out as much play as possible of the whole assembly .
There was still some movement but not as much , so I slipped a turned thinner washer between the hand wheel and the spindle boss which moved the screw thread back a good 1.5 mm .
The final fit was achieved after I hand rubbed both sides of the turned down packing washer on a sheet of thick glass using wet & dry paper plus water so it became nice and mirror polished. Washed it off in meths a couple of times & dry cloth polished it to remove any trace of abrasive then greased it with a dab of silicone & copper grease for the final fit .
This helped tighten up the spindle action tremendously ... from , " Look at it and it moves almost on its own" , to needing a gentle turning force to get it moving nice and smoothly . Getting that end float down to almost zero took several test & trials.
Remembering to turn the feed wheels back beyond where you normally would to take up wear in the acme threads before you start the cuts took a wee while to get in my brains muscle memory ..now it's second nature .
