Terrible artifact in Field Potential recordings

[Tama81]

Hi guys,
I perform field potential measures in nucleus accumbens slices. During recordings I apply a square current pulse (200us) every 30 secs and, for each pulse I usually observe a presynaptic signal at 3ms and the fEPSP at 5ms after electrical stimulation of presynaptic fibers. But since some days the current pulse determines a huge artifact covering the presynaptic signal at 3 ms (usually the artifact starts at 1,8 ms and ends at 2ms). I'm sure that it's an artifact because it also appears even when there isn't any slice in the recording chamber. The stimulation electrodes are new, and the recording electrodes have a tip diameter of 1-2 um; unfortunately I can't measure the resistance because my tester is broken. I use a lowpass filter at 2KHz and no Highpass.

Please help me!
The deadline is near!!!!