PURCHASE ORDER NUMBERING OUT OF WACK

I believe i may have asked this previously but here i go again as I really need a solution or an answer please.

When we raise an order and one person has already selected a number and is filling out the order and another person is also working on the same thing when either of them finally go to email or save it the number will change according to who saves first. Have no idea why this happens but it's so frustrating as one person may have manually used a number but it will automatically change when it is sent. 

This also occurs with Sales as well. When two are adding a quote or order at the same time the numbers will change according to who saves it first regardless of the number that is already on their order. 

I've also noticed that if a purchase order is already saved with a particular order number when it is then changed to a bill the purchase order number changes to the next in the auto system rather than keeping the number that it was already saved under. 

This is s major flaw and is creating a number of double ups and mismatching of purchase orders to invoices and sales. 

Why is this happening and what can be done for it not to occur.

Also as there is no area to write a job number without having to creating a new job every time (as we don't need it recorded as it's too time consuming and we don't track it) we use the comment section but have noticed that when you raise a new order random job numbers appear in the comment section even though it's a new order - why would this be happening? Again very confusing for a supplier and when we are marrying a job to the purchase order.