How do I interpret these autocorrelograms?
To answer your specific question:


  For instance, if I see a peak at say, ~ 750 ms, does this mean that with a lag time of 750 ms, the firing pattern is more constant/predictable, or does it simply mean that I have more inter-spike intervals of 750 ms?


You are calculating a histogram of inter-spike distances, binned at 50 (in the code as written). Obviously the spikes are only zero time instants away from themselves which is why the high peak at zero.

If there is a peak in the histogram at 750ms, it does not say anything about repeated inter-spike intervals of 750ms. That is, just because there is a second spike 750ms after the first spike, it doesn't necessarily mean there will be a third spike 750ms after the second.

It just says that, over time, there is more likely to be a spike 750ms after any given spike.