I take my hat off to Sam Courtney at Wanhao UK. My 4DS was very poorly and not printing well. Also it was resetting itself at random during a print.

Sam gave a lot of advice and also offered to send parts out, with the promise of a refund if they were returned as not needed (i.e. didn't solve the problem).

In the end I found two faults.
Muck in the card reader was possibly causing mis-reads and hence the resetting and freezing. This appears to be sorted with the air blower.
A faulty X motor cable (broken conductor - it is the only cable that flexes). I had a spare, so replaced it.

Now it is as good as new and is happily printing.

Cheers to Sam and Big Thank You!

Regards,

Rob