Clarity of buttons labels while creating questionnaires

I am in a predicament and would appreciate any help in solving this puzzle.

I am creating a questionnaire in a modal based interface, where there are three buttons. Next - saves, and bring in the next question template. Previous - saves, and bring in the previous question. Save - saves the questions, and closes the popup.

I am afraid the user will have no idea that the Next/Previous buttons are actually saving their questions before taking them back and forth. And what if my user keep clicking save, before clicking next/previous? Also, another functionality that 'save' has is that after saving, it is auto closes the popup.

One option is to call it 'Save and Close', but how do I reassure my user that everytime they click Next, or previous, their questions are indeed getting saved.

Any advice would be much appreciated :)