It sounds like your system is not able to migrate everything it
needs to migrate, to the other cores.
Things I would check would include:
1) BIOS APIC versus PIC setting. You want APIC.
Verifiable from Device Manager, View:Resource By Type,
Interrupt Request (IRQ). Check to see if interrupts go up to at least 23.
If limited to 15, that's PIC in the BIOS doing it.
2) BIOS MPS specification. This is not available in newer motherboards,
because it's always turned on. On an older motherboard, you can have
MPS (MultiProcessor Specification) 1.1 or 1.4, and you'd select 1.4.
Probably verifiable, as in (3), whether it's working.
3) In the OS, Device Manager, Computer entry, Properties, you want
to check that a good HAL is in place.
ACPI Multiprocessor PC
That should enable usage of IOAPIC, so that hardware interrupts can run
on any core they want. If you don't have IOAPIC, for any reason, then
I think Core0 handles interrupts. That kind of sounds like what has
happened to you.
What's the make and model number of the motherboard ? So I can
go check the manual...
This all sounds rather unlikely, but I hope that gives you a
starting point. Maybe it was the priority setting you did or
something, but that would be an unexpected result.
Paul