OK this is from a complete outsider on this issue since I have yet to even purchase a VX, but classic GTOs have had this issue for years.

What we have found over time is a multi pronged attack. Of course proper cooling system (i.e hoses, radiator, fan, thermostat, coolant) looks like you've all tried this. We also discovered that the fan shroud itself made a HUGE difference on how the fan pulled. But in some cases that STILL was not enough, after further investigation it was discovered that fuel and timing were also a big part of the equation. With the inability to get leaded high octane gas it was causing detonation in the combustion chambers raising the temps, as would an engine slightly out of time or having a bad advance setting on the distributor. Has anyone looked to see if over time the timing belt stretches just enough to let it get a tad out of time, or if the ignition system needs tweaked? Just an outsiders observation.