Very High speed response AC coupling
You need to make the cutoff of your high pass filter higher.  The question then being whether or not you can make it high enough to eliminate the changes in the "DC" without also destroying your AC signal.  

You will need a sharp filter with a cutoff between 50Hz, and the highest frequency you expect to see on your "DC" signal.  A simple RC highpass probably won't be sufficient.

I assume you are still working on this system.  The same solution discussed there would be useful here.  Use a relatively weak high pass on the analog side, then do a very sharp highpass digitally in Matlab.  As also mentioned in the other question, a non-causal filter will help eliminate the slow walk to the correct DC value - here it will help remove the slow walk to true DC removal.