I'm beginning to think at a cpu issue, but if you measure the Rx/Tx +/- print lines at voltage and ohm, you should measure a issue with it.

Maybe you see some voltage drop, or strange ohm value.

In any case, see this information,
http://jb8a8f8.com/support/index.php?attachments/dm800hd-networkv3-jpg.58/

And do some measurements on the network circuit.
But remember, the ohm values are measured between the ground and the measuring point.

However, to do a real ohms measurement for checking the cpu connections Rx/Tx +/-.
You need to remove the network connector from the motherboard.
And also the four resistors R132 to R134.
If you do that, than the ohm value from the Rx/Tx +/- should be in the same Ohms range.

But, it does not have to be a cpu issue, the network connector has internal coils and some other parts.
Suppose that one coil connection has been broken, than you get also this symptoms.

In that case you must replace this network connector for a new one.

Other thing that it could be is one of the ceramic caps C121/C122
But if one of them is shorted, you should notice it by measurements.