Without the exact comp settings and your preview settings we can't tell you much. You could have accidentally messed up the framerates or something like that...
This comp was 1920x1080, 23.976 fps. Because I was having this issue, I created a new comp with absolutely nothing in it. Then created a line of text and had it grow using two keyframes. It still presented the same issue.
This computer is connected via 10Gb Ethernet to a shared storage so I simply opened this comp on an identical machine (We have multiple edit suites with this same setup) and it did not present the same problem. It indicates a local issue, but as I said I dumped the preferences and it still happened. I'm completely dumbfounded.
Let me make sure I understand the situation.
It happens on one machine in both CC 2014 and CC 2015, but not on other machines on the same network when opening the same project file?
It happens only in preview, not in render?
Check your preview settings to see if you've got it set to skip any number of frames.