How to use a filter to reduce data set from 512Hz to 12.5Hz (80ms)

I have some data (a few minutes worth) which is is recorded at 512Hz, so a data point approximately every 2ms. I need to filter the data so that i get the data at 12.5Hz, so a data point at every 80ms. This is to remove the spikes in the data.

I have tried used a butterworth filter, which includes filtfilt, but this ends up with NaNs, this I think is because the filter can't reduce it to that low a frequency.

How do i reduce the data set to only pick up spikes lasting over 80ms?

I'm using matlab R2010b 64bit.