wild guess... your hosting company (or you) changed the email setting.

i use authorize.net, however i have modified the timing of the transaction submission due to clients needs. so i would not get into this situation. however, when the sending of an order email fails, it screws up the whole order process. see:

https://www.zen-cart.com/showthread....41#post1311441

frankly, i'm not convinced that this is the problem, but you were asking for anything else...

it does seem like a network problem. especially when you say the order ID gets assigned to the next customer's orders. i have had experiences similar when there are problems with queuing on other systems...

these are always the trickiest to figure out.

good luck!