Filters performance analysis
OK, doing a design of a Butterworth high pass filter with a cutoff frequency of 8Hz and a sampling frequency of 325Hz using scilab by:

 bw = iir(4,'hp','butt',[8/325 0.5],[0.1,0.1]);


then the filter is:

                                  2  
0.8168388 - 3.2673552z + 4.9010328z   
               3            4         
   - 3.2673552z + 0.8168388z          
------------------------------------  
                                   2  
0.6672256 - 2.9384352z + 4.8677204z   
               3   4                  
   - 3.5960396z + z                   


and the frequency response looks like:



So I'm not sure what you're doing for the design, but it doesn't appear to be high pass.

One thing that may help is if you could share what your data looks like (say 256 or so points?).