. You have pretty much done just about everything to pin point this.I'd say to take out the old re-formed filter can's and put in some new F&T's. What you describe is strange,but the filter cans might be the culprit, since you get 'target' B+ after running it for awhile, but, as soon as you power down you get FAST filter can drain, only to later power back up, and find that the B+ is high.
Then it hit me. "Hey dumby, change your HT filter." I'll give that a shot. Thanks Andy.