If you have not done so already, you should take advantage of Bill's offer to send you a line filter. I know that the PWM motor controllers generate a lot of EMI, based on my experience with putting DRO's on my mill. It took some good shielding braid to keep the noise from messing up the readouts. An arc-sensitive breaker could well interpret the EMI as something dangerous and open up. $45 is a very reasonable price. As a matter of fact, Bill, where did you get that one -- it's something I want to consider, too.
As to why other apparently-unrelated circuits could be affected, the EMI also may be coupling into them via stray inductance. All it would take is to have the noisy line run in parallel with another one for some distance, and you've got a transformer, especially for noise "spikes". Shielding is not particularly effective for noise created by current spikes so metal-sheathed coax can be susceptible to this as well.
In a situation like this, I would not mess with ad-hoc fixes like moving connections around in your breaker box. One small variation down the road could make the problem come back. Take care of the problem at the source, and you've solved it for once & for all.
Mark
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