If you’re always using the same passband, design your filter once (use buttord to help define it), convert it to second-order-section (sos) implementation, save the sos structure in a ‘.mat’ file, and then load it to use it with filtfilt in your file processing function.
I couldn’t reproduce your error, but then you said it was intermittent.
Example (run this once, possibly in a separate file):
Wp = [9600 10560]/(fs/2);
[n,Wn] = buttord(Wp, [0.925 1.025].*Wp, 1, 50);
[b,a] = butter(n, Wn, 'bandpass');
[sos1,g1] = tf2sos(b,a);
save('Filter_1.mat', 'sos1', 'g1');
in your filtering script:
load('Filter_1.mat');
y = filtfilt(sos1,g1,x)
