Although it is in the Blitzortung forum, there are some tips
here that may be helpful. There are some generic ideas that could apply to your Boltek. (the frequency range and "signals of interest" are the same between Blitzortung and Boltek, same physics apply)
One basic method is to use an AM radio set to the low end of the band <500KHz and see if you can find where it gets noiser. Are you using shielded CAT5 or CAT6? I know that Boltek recommends STP (shielded) CAT5.
The PCI version makes it harder to isolate, since if you had the LD-250 or LD-350 you could power it from a battery and shut off all the power in your house. Even so, if you can leave just the circuit breaker/fuse for your computer active to power the PCI Boltek, and shut down everything else that shares the computer's circuit it might help isolate the problem. At least you might get some clue about if it is
inside your place or outside.
If you can see if there is anything that operates nearby during the noise times, like neon lights, welders, motor controllers, etc. it might help track it down. Also, shut off all of your other circuits at the power panel. I would look at potential noise sources that agree with the directions in the map plots.
Good Luck in finding the problem. This might also be a useful resource:
ARRL EMIGreg H.