I have had this problem too
You will find out if its the bushes by sitting on a chair and put the hoof between your legs, then with one hand holding the top of the spring mount and the other on the bolt thats behind the foot plate ( where the down tube and foot support meet ) and push and pull it forward and backwards ( aka away and towards you with the spring to either your left or right.)
The solution to fix it is to bond the bushes into the frame
http://www.projumpforum.co.uk/index.php?topic=1271.0if you still get movement then ask jason about his brass bushes ( but be warned they cost a lot ( his set cost round £40+ just for the brass metal.. then he had to spend time machining them down into shape ))
Chris - Binop9