What is the year and model of your mac? Also how full is the hard drive? How much memory is installed? Is this a recent occurrence or has this always been the case?
I tried exactly what you are doing and had no issues. It is possible the install of powerpoint is corrupt. It is also possible that the video driver is at fault or the video card is going bad. Or the hard drive too full and not allowing enough room for swapping.
If it were me - I would at minimum run disk utilities and check the hard drive and permissions by restarting and holding down the Command + R keys. I also love
Onyx for cleaning up my system caches and getting the junk out.
I would suggest upgrading to Yosemite 10.10.5 (not saying it will help but it might fix a video driver corruption.) If that did not fix it, would reinstall powerpoint. Or you could reverse it and reinstall powerpoint first. This is all assuming it isn't because the hard drive is too full.
Let us know what you find out.
Lisa