Ummm... I've read what you posted, gave it some thoughts, I looked at the FSM, goggled.. searched.. and thought.. and this is all I could come up with..
Just FYI..
Most of what I know about fluid dynamics comes from dealing with my swimming pool
But anyways..
As that the bypass line feeds the thermostat, that also means the bypass circuit and rad feed into the water pump intake, which is blind as to the source of coolant. Any fluid will flow along the path of least resistance, that means that if the rad is not flowing properly, the intake to the water pump will draw more coolant from the bypass line than normal. The position of the thermostat in the engine ( at the intake to the water pump, as opposed to the block / head exit ), and the bypass circuit, as designed work just fine, but will not tolerate blockages in any aspect of flow. From that you must keep the revs up, would seem to indicate, to me, that the additional flow through the system, caused by the higher RPM / pressure from the pump, stops localized boiling.... until you're at idle in your driveway..
As I've noticed from reading other posts in this forum, the failure of the head gasket on these engines can be hard to spot. Traditionally, a head gasket failure is usually fairly easy to spot... white smoke out the exhaust, difference in compression between two adjoining cylinders, coolant loss, etc., However, it would seem, that an SVX engine can have a head gasket problem that is not so readily apparent. And therefore, must be definitively ruled out before looking elsewhere in finding an issue, especially when it comes to a cooling problem.
I do have one question... Did the problem begin with a bang? Or, did it slowly begin to manifest itself... And if so, over what time frame?
Just as an observational note.. I have a temp gauge with no "dead zone".. and I've reconciled the gauge against the ECU readings with an OBD 2 reader. I've watched closely how the cooling system reacts to load, road speed and ambient temperature. I have no idea how old the rad in mine is ( it's a 97).. At 0 C ( 32 F ) it will stay at the thermostat opening point and not budge at anything over 40 MPH ( 75 KM/H ) ... below that, it might rise a bit, but not much. Say for example, at a light, it slowly rises towards the fan turn on point, how quickly it will return from a higher temp to it's base point is entirely dependent on road speed.. If It's at almost at the point of turning on the fans, and I hit the freeway, it's less than 30 seconds to drop it back to 185 F. I tried revving it ( locked in a lower gear or held them longer while driving / accelerating ), and couldn't see any difference in the temperature changes. I do have the rad surround installed, and the under engine shroud / pan installed..
Don't know if that helps at all.... but best of luck!!