throughput and interrupts

Hello.
I am completely stuck. I have a router, that rules three subnets. At first 
there were Allied Telesyn's netcards with realtek chipsets. I am using 
ipfw+natd and I had like 3,5 megabytes per second with 20 per cent of 
interrupt load. I had "discarded oversized frame" too often so I've replaced 
two main cards with 3Com 905 and now my xl0 and xl1 never discard frames. But 
I saw no speed boost and interrupts were also very high. I have discovered 
some IRQ conflicts and removed them, now I have single device per IRQ, but 
still i have near 20-30 % of interrupt load, when i download something 
through this router (and natd takes rest % of CPU). Some people advised me to 
use polling, so I've built kernel with HZ=1000, polling and tried it. 
Interrupts grew to 40-50% and speed decreased to 200-300 kb/s, so I had to 
turn the polling off.

I just don't know what to try, to gain at least 9 mb/s, because this is what I 
get at home with the same cards, the same processor, with no polling and 
throgh the same provider.

Best regards,
Bachilo Dmitry
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С уважением, Бачило Дмитрий
Руководитель отдела системной интеграции
ООО "Компания СоЛинк"