If it's sticking enough to stall going that slow then this should be easy to find.! It sounds very much like very dry or knackerd lead nut to me with it coming on after time.?
Could be a tight bearing
Do this in a methodical order and you'll track it down quick.
Start by disconnecting the lead nut and running gantry up and down feeling for binding.? If it does then loosen bearing mounts and again run up and down seeing if it lessens.? It could just be an alignment issue.? tighten up and if gone then happy days.!! . . If still sticking then chances are it's a bad or dying bearing.

Check the lead nut for wear or crap embedded in it. Run it up and down checking for tight spots. Unfortunately if it's worn or tight spots then nothing much can be done.!

One thing I've experienced with these machines is the wiring is rubbish. Recently I repaired one where the Z axis and Y would change direction for no reason which turned out to be stepper wires. New shielded wires and machine ran a lot nicer and stable.