Rotating ball nut, Ballscrew Pitch, RPM limits, and my particular build pickle

[Bongo]

Hello All,
So I started building a 1.2 c 2.4m bed CNC a long time ago. It's a bit unusual in a few ways, the gantry being a frankensitne mash-up of a scrapped stairlift extrusion, steel pipes, epoxy concrete, and home cast aluminium end plates. The machine build has not proceeded the way I see most builds happening (ie a sensible CAD design that gets tweaked BEFORE construction). Thats not to say I had no design, just it's morphed quite a bit during the build. I guess you could say I am quite a thick-headed so and so, that needs hands-on experience to learn anything. Ok that's enough background for now - the dilemma, and reason for the post...

The x has two 2.4m long, 5mm pitch,32mm dia. ballscrews, which I have had for at least a year before finally getting round to fitting. They are from China (linearmotionbearings2008 on eBay) and are both a bit bent (approx 1mm off true bow). As I was going the rotating ballnut rout, I thought it wouldn't be such a problem, as the sideways force required to straighten them is not much over such a length (indeed the sag by gravity is noticeable), and I would be able to tension them a lot as the screws themselves are stationary and don't need to worry about over tight angular contact bearings.

Here's one screw rigged up to the rotating ballnut assembly I have cobbled up:



Things to note: I am testing this with just one screw - I haven't yet mounted the slave ballscrew on the other side... There is very little (no detectable by eye) racking in the gantry, even though I'm driving it from one side only, which seems promising rigidity wise.

I think the 5mm pitch was a mistake! I thought that with the 400W servo (3,000 max RPM) and no gear reduction, I would get reasonable travel speeds (3000 x 5 = 9000mm/min). 9000mm/min sounded ok to me - keep in mind I'm a complete CNC newbe. What I didn't consider is the ballnut's limitations. From the word go the nut has felt a bit 'crunchy' - or at least not smooth, and also sounds a bit rattly - despite my greasing... I didn’t get them on video but, there have been a few ‘jams’ that have stalled the servo. When I turn by hand I can verify that yes, it is just the ballnut ‘catching’, and with a bit of ‘back and forthing’ I can sort of feel it past the jam. The catches have happened at relatively low speed. When going faster I guess the servo just powers through them.
So options as I see them:

1. Ditch the cheap ballscrews, bite the bullet and buy some good ones of 10 or 15mm pitch (suggestions for what would be ‘good’?).
2. Continue on, mount the other x ballscrew. My feeling is this might hide the problem for a bit... In fact, writing it out here makes this seem a very poor choice?
3. Try some ballnut fixes? push more grease in? attempt disassembly and look for swarf or other potential jamming culprits (how possible)?
4. Something that I haven’t thought of that may seem obvious to you lot??

Ok thanks a lot for reading, please share your thoughts :)