Soundstage there/gone/moves - poor current ????

It sounds like it could be an intermittent connection. See if you are getting sound from both tweeters when the problem occurs. A lack of highs can cause the soundstage to drop to the floor. Also, check your speaker cable connections.

I agree with Nighthawk - it sounds much more like a system problem (including line, breaker, cable and connection integrity) than a line conditioning issue.

Judging by what you've swapped out already, I'd go systematically through cables and power cords next. Also check outlets on a different current loop if you're able (by using a heavy duty extension cord and forget the additional noise). I know you've done a good deal of troubleshooting already but have you tested simple things like (a) the continuity of your outlets and power cords with a line continuity tester and (b) checked for really corroded connections anywhere along the power/cable routes? I've found stock power cords that tested as having an open neutral(!) yet still conducted (some) current this way.

In my area, the city power regularly cycles on a daily basis from 120 volts to below 114 V, and a few towns away, people report city voltage above 130V. Just more issues to look at when you're on a gremlin-chasing spree.

Make sure nothing on AC cirrcuit as stereo is turning on/off/ emitting alot of RFI. Maybe something budget friendly like AudioPrism Quietlines, Enacom AC Filters, power conditioner or power regenerator like PS Audio 300 ( kinda pricey) will do the trick.

You also might want to check out acoustic panels made by ASC, Echo Busters, etc. Or construct DIY acoustic panels as mfrd. panels can get expensive real fast.

The surest way to rest your mind at ease, if its possible/practical, would be to have a couple of dedicated lines installed. If that's not possible, I would check out what outlets are on the circuit you are using. Many times these outlets are daisey-chained from one to another. If the outlet you're using is at the end of the chain, this is not optimal. Every outlet needs to be replaced (w/high quality ones-no push in wire types)leading up to the outlet you're using for your stereo. Also, ask the electrician if it's possible to pigtail a wire of the splice to power the outlet. Many times the cheapest outlets are used and the easiest way to continue the run from one outlet to another is to simply push wires in the back of the outlets. Dedicated circuits are the way to go. However, if the feed going into your service box is not up to date, i.e., old/undersized, get a new hobby or move! You'll could end up spending a fortune and not getting the results you want. Unfortunarely, the AC power is so polluted nowadays and that computer in the house isn't helping one bit. You're right, this is one fustrating hobby. Acceptance is virtue. Good Luck, Bill

As Nighthawk suggests, I would investigate whether an internal connection in one of your loudspeakers, maybe at a driver or in a crossover, is somehow intermittant.