Yup! Unless it is tied to the jack ground lug, it doesn't count. Can be through the pots if they are grounded, but has to all be connected. Do they make metal covers for P-90s (I've seen the dog-ear ones). If they do you could put them over the top and solder a ground wire from the covers to you pots to ground the shields. You mention the dirty power, but make sure you have a full 3 wire outlet with ground or your going nowhere fast. If things are as bad as you say there is always one of these. A nice isolation transformer to put between you and the power company. Spikes, glitches and hums get eaten up in the coils of the transformer, essentially isolating you on a separate AC circuit from the mains power. Most have some sort of EMI filtering as well.