KeStallExecutionProcessor and KeAcquireInterruptSpinLock and HT performance

I have the following code:

Irq=KeAcquireInterruptSpinLock(…);

KeStallExecutionProcessor(25);
KeReleaseInterruptSpinLock(…);

It works fine on a single-CPU XP, but I get the data loss on a HT machine.
I suspect it might be because it stalls the ISR, but why do I not get the
result reversed? IOW, single-cpu gets the data loss but HP is fine…?

Thanks

AH