Make sure you have the Rings round the right way, there is a copy of theoriginal ZX5 handbook on this site under reviews.
I expect others with disagree as Roger already has, but the things that have made it easier for me are
1 - Lots of grease
2 - Make sure all parts are ckean before you start
3 - tignten the diff up until tight and then got back about an 8th of a turn.
I had the same problems as you to start with, but i did one today and it went together fine.
Keep trying
