Keynote: 14 second delays before transitions when playing

Hi,

I'm putting together a Keynote presentation for an event. When testing/playing back the file, I'm getting a huge 14 second delay before transitions between slides occur. The timer stops, and then continues, together with the presentation playing, 14 seconds later. I've viewed the same presentation on a colleague's iMac and the presentation is seamless - no delay (unless specified by me of course!). I use Keynote a lot, so I'm confident that these delays are not of my own making.

I'm using an iMac (Retina 5K, 27-inch, Late 2015), with a 3.2 GHz Intel Core i5 processor and 8GB 1867 MHz DDR3 memory.

Keynote version is 6.6.2

Both Keynote and the system software is fully up to date (I updated everything in the hope this would fix the issue).

Any suggestions/solutions would be most welcome as I'm pretty flummoxed.

Thanks

Gareth