yellow LED turns off

Hi!

I'm investigating following problem with my RTL8139 chips:

normally all networking works just fine, but...
it happens once per month, but sometimes once per day, that yellow LED
indicating LINK-ON turns off. when ping'ing the machine I don't get any
responses.

the 100% trick is to plug off and on RJ-45 into port:(
but since it's embedded 24/7 system it's not so easy...

first, I thought that this can be matter of kernel driver, so I changed
it from 8139too.c (linux-2.4.18) to old one (linux-2.2.19). It seemed
that this was the case, but problem started to wake again.

in middle time the was idea of some kind of electric shock, that chip
receives by the cable...

another idea was some kind of power management inside RTC chip, so I
studied realtek tech specs and source code of both drivers.

I saw rtl8139_chip_reset() routine in driver. now I wander if using it
like a watchdog could be the way to solve the problem -- it's just
software reset. trick with plug out/in cable suggest that this should be
rather hardware problem.

monitoriong my few embedded linux routers shows that it's not case of one
buggy motherboard -- LED turns of in near every one, but with totaly
unpredictable frequency, so it's hardly impossible to trace this.

there is one RTL chip which supports 3 LAN.
device is: http://www.advantech.com/products/WEB-2143.asp
but they seems to ignore my questions:(

what's happening inside RTL chip when I plug RJ-45 into? is it possible
to emulate it by software?

maybe any other ides, experiences?

Thanks in advance.

-- 
Piotr Trojanek