Frankly, I've come pretty close to running out of ideas. I doubt the boost control solenoid is the problem. That would be more likely to prevent you from getting into boost. The actuator is that black cylinder that has the vacuum fittings on it.
Unfortunately, doing the boost bypass was not the solution to the problem. Some time after I posted that, I found that the reason that the car was no longer doing it was because it was not going into boost. Rather than leaving the bottom hose unconnected, I had plugged it which kept the actuator from being able to allow it to go into boost. The "real" solution was what did not work for you. Cleaning the grounding strap connection from the battery. I cleaned it up, scraped off some paint, put on a star washer that would bite into the body better, and gooped it all up with dielectric grease. That was maybe 8 months or so ago, and the problem has not returned.
As it only happens as it enters boost, doesn't necessarily point to a boost control related problem. Lots of things can affect it as it goes from sucking the air in, to having it blown in.
To be sure - did you clean up the positive sides also? The battery, the connection to the rear and under hood fuses boxes, as well as being certain that the bolts holding the boxes down are good and snug?
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.