Sales Orders and Invoices not staying sequential

Just noticed the when raising sales orders it's not staying in sequence, records show it has been happening for a couple of weeks but no one noticed as the numbers are very similiar excepting the first three digits i.e., 00 or 000.

I have read other forum posts and understand how it can happen and it the past overriding the sales order number fixes the problem. However this time it will stay in the correct sequence for a few sales orders... then it reverts back the incorrect sequence .

The correct sequence number is 00121223. The incorrect sequence number it goes back to is 00012258, it would be many years since we were using this range of numbers. 

Any help would be appreciated.