Spoke to Porter Cable service this AM. Very nice lady was surprised and said she had never heard of this problem with the vacuum. She was unable to explain it, but insisted that I not use the vac again, as she is sending me a new vac to replace it. The more I thought about it, I concluded that neither the 590 (unplugged at the time) nor the vacuum is grounded, the Porter Cable has a two prong plug.while I am grounded (on my back porch slab). Probably could be resolved if the vacuum had a grounded (three prong) plug which it does not. Anyhow, I am pleased with the response, however, not sure that it will not happen with the new unit. Will test and report when I get the replacement.