One thing to try to reduce the noise is to isolate the computers PSU, especially if you are using a laptop, the crappy power packs that come with laptops today are quite useless at filtering noise on the incomer which can then play havoc with all sorts of timing gear, we installed a ups for using our MyLaps loop running from a small genny, then we had similar issues running from mains power with our miniZ loop, noise on the ground plane. Any crappy UPS or Line Filter will ake a world of difference.