Invoicing

Randomly when I am preparing an invoice, after entering an item in the first column I'll tab to the next field, but the curser will drop to the next line instead.  It will continue to do this for every every line of the invoice.  But when I start a new invoice, the curser will behave properly again.

It's very annoying as some of my invoices are many pages long and I have to remember after entering every item to tab back to the correct line.  This is really slowing me down.  Any ideas how to fix?  I haven't been able to determine the circumstances around when this occurs.  It's just random!