CPU load inhibits CAN interrupts

[Krzysztof Witkowski <Krzysztof.Witkowski@mobile.put.edu.pl>]

I have an Ubuntu box with 3.2 kernel, CPU with 2 cores and PEAK-PCI card 
(SJA1000 controller attached via PCI bus)

I'm testing receive capabilities of the card on 1Mbit network using 
peak_pci module. It can handle about ~4000 packets per second, the 
corresponding interrupt is also raised ~4000 times per second (as 
/proc/interrupts shows) and it does not cause high CPU load on the system.

However, if I generate artificial CPU load with stress command:

    chrt --idle 0 stress -c 2

the Receive Interrupts from SJA1000 are no longer raised at all and 
therefore no messages are received. At the same time network card (e100) 
on the same IRQ is working properly.

Why CPU load inhibits hardware interrupts and what can be done about it? 
Or should I suspect hardware malfunction?