Flush the disk caches and turn off using them. I wanna bet it fixes your issue...
As Mylenium said this is most likely a cache issue. When doing this kind of repetitive keyframing I usually turn all caches off and purge often. You should also turn off cache indicators in your timeline. You'll find the setting in the Timeline Options menu at the top right corner of the timeline.
Thank you very much for your responses! Dave LaRonde, I'll be sure to give more spec info up front next time. Mylenium and Rick - makes sense, I'll keep that in mind going forward.
Luckily, and I totally forgot this in the moment, I turned on Time Machine that morning after months of having it turned off for hard drive space reasons - and so I’m back in business!
Thanks again Gents