RTL and power management

[Arnold Radtke <Arnold.Radtke@Ferrari.DE>]

scenario: interrupt every 125us and very short ISR 

ISR:      rtl_no_interrupts(state);
          t=gethrtime();
          ctrl = inb(..);
          ctrl|=STROBE;	
          outb(..);
          ctrl&=~STROBE;
          outb(...);
          diff=gethrtime()-t;   
         rtl_hard_enable_irq(irq);
          rtl_restore_interrupts(state);
 
          profiling runtime of ISR with gethrtime at beginning and end
of ISR.
          runtime normally about 5us, but sometimes ISR runs about 195
us.
          All interrupts are disabled. 

Does anybody know what happens in this 190us? 
It's perhaps power management?
I compiled a 2.4.0-test1 kernel without power management, but couldn't
disable it generally in the BIOS of my PC. Now I will try it on a PC
without power management.

Thanks for any ideas.

Arnold