Check the parallel cable from the PC to the bob, is the inner shieldings fubar, even letting it cross the power cables will do that. Its sounds like your problems are to random for it to be a physical interference problem. but interference on the parallel cable would do all of the above.

you can test this by keeping the cable as far away from the others, but if its the actual connector ends of the cable, it might not show. then place it across some see if its worse.

If not, ( i suspect it is the above ). check the PC to see if anything is on the same IRQ or interrupt as the parallel port.

I say the above from experience.....