Just an update here.  I have now rebuilt the cross slide drive with direct coupling of the motor to the leadscrew and things look much better, so I hope I'm back in business.  Thanks for all the input.

John.

 how good is the Myford bed .Is it worn .Myfords have a habit of shying away from work with there flat bed design .  I used to have one and found that a couple of zero cuts helps . I now run Hercus/Southbend . And find  heaps better . give it a go

I got the lathe new and it hasn't had much wear.  It was cutting undersize, not oversize, so zero cuts wouldn't help?  But thanks for the suggestion, another thing to look at if still dodgy.