If you create a timed presentation that automatically advanced after a certain number of seconds -- usually done in a kiosk situation where viewers view the presentation on their own -- and include hyperlinks to allow viewers to review the presentation, after they click a hyperlink to go backwards, the presentation stops advancing.

Apparently, PowerPoint was designed so that if you hyperlink backwards to a slide that you've already viewed, automatic timing stops.

Create duplicates of each slide. Then set the timing for the first copy of each slide to 00:00. Because it's an exact duplicate of the following slide, you don't notice the duplication at all. (You might think that you wouldn't see any slide set for 00:00 timing, but you actually see it for a split second, so if the slide is not an exact duplicate of the next slide, you'll notice it.) Then hyperlink to the first slide of each pair of duplicates.

I found that the easiest way to do this is the following:

Complete the presentation, including the timings.

Click each slide (in the outline pane on the Slides tab or in slide sorter view). Press Ctrl+C to copy it, and immediately press Ctrl+V to paste it. You get a copy right after the original slide. Select the first copy of each slide, choose Slide Show>Slide Transition, and change the timing to 00:00.

If you have added sound to any slide, remove the sound icon from the first copy of each slide (the 00:00 ones). You wouldn't want the sound to start to play twice.

Create the hyperlinks. You can copy and paste them throughout the presentation. What's important is that the hyperlinks link to the first copy of each slide, the one set for 00:00. Because it has no timing, PowerPoint won't stop the timing when you hyperlink back to it.

If you use Action Buttons, set them for hyperlinks. Instead of setting them to Previous Slide or Next Slide, in the Hyperlink To drop-down list of the Action Settings dialog box, scroll down to Slide. Then choose the first copy of the previous or next slide.

If you later change a slide, be sure to delete its copy, copy and paste again to duplicate it, and change the first one's timing.

If you use the Slide Numbering feature to number the slides, the numbers won't be correct; instead you need to manually add text to indicate the slide numbers.