Custom navigation inside quiz

Hello,

I have a quiz set up that consists of 86 questions. On the slide master there's a number variable that shows the student which question they're on currently (called 'qId' - with the 'next' & 'prev' buttons altering it respectively (+1, -1)). After receiving complaints from students that it takes them forever to get around the quiz, either while reviewing or before submitting the final question, I added a "Return to question #1" button that would basically pop them back to the slide draw and the first question.

Recently I have been trying to improve this navigation while still holding on to the randomization we get from the question bank - and this was done by splitting each 10 questions into a separate bank, and adding custom "+10" and "-10" buttons, which would - you guessed it, jump to each slide draw (according to qIds current value) and alter qId.

This is still a work in progress, however there is one big obstacle I can't seem to overcome:

When I'm at the first of each 10 slides in a question bank and I press the 'Prev' button, I'm automatically transferred to the previous question bank, meaning the latter's first slide (effectively jumping 10 slides back instead of 1). 

Any help or alternative methods would be greatly appreciated.

Thanks in advance,

Erez