Stock Quantity Issues

Wondering if anyone else has had this issue.
Running 1.5.6 and the stock quantities are proving to be a major problem.

Ive just updated the stock for an item that was out of stock. 2 mins later its back to 0.
Ive repeated the process 7 times now and it keeps jumping back by itself.

I thought I was doing something wrong but our staff also report the same issues on a number of products.

How can this be happening? I know these items have not been purchsed online by customers

thanks
G