I'm using a bipolar stim electrode and am getting bubbles on the tip with every sweep down to 0.1 mA and sometimes below. I've cleaned it multiple times but it seems to be getting worse and worse. I'm using a biphasic stim box which helps a little. I'm stim'ing in either cortex or callosum and find best responses around 0.4-0.5 mA, but this is in the bubble range. Any ideas why this is happening?