It looks like C and D chips have issues. Next step is to find where they are located on the board and if replacements are available.

It appears the memory chips are "HYB18H512321AF-14". Unfortunately, no one sells this exact model anymore.

Options are ;
- Try the identical model but slower speed "HYB18H512321AF-13". https://www.aliexpress.com/item/1005008640348104.html (this would involve dropping the memory clock to 675 MHz).
- Try a slightly different model with identical specs (I think) https://www.aliexpress.com/item/1005007174051898.html
- Find other cards that used "HYB18H512321AF-14" and harvest from that. Maybe a 7800GT PCIe ?

I think the memory order goes like this for older cards. I've highlighted the bad chips. The MATS log says bank 0 (EB0) and bank 1 (EB1) of D has errors as well as C0. In this situation, that is referring to a single chip. On something like a 7800GTX bank 0 is on the front of the card and bank 1 is on the back of the card, two different chips.

Since D1 has millions of errors, a reflow of the chip is probably not going to work. A reflow of C0 might work since there aren't too many errors. In any case, I'm going to replace both chips since it's such a rare card.

I replaced D1 and reflowed C0 but now there are more memory errors. I think the next step is going to be reflowing or reballing the core.

1  LANE    EB0 READ ERR EB0 WRT ERR  EB1 READ ERR EB1 WRT ERR  UNKNOWN ERR 
2---------- ------------ ------------ ------------ ------------ ------------
3FBA[31: 0]           0            0            0            0            0
4FBA[63:32]           0            0            0            0            0
5FBB[31: 0]           0            0            0            0            0
6FBB[63:32]           0            0            0            0            0
7FBC[31: 0]      753975     24244283       945925     24040487            0
8FBC[63:32]           0            0            0            0            0
9FBD[31: 0]     7093405     18112739      6987330     18218174            0
10FBD[63:32]          98     16817461          194     16816754            0

Voltages?
Tried underclocking?
Checked smd caps nearby for shorts?

Mats is brilliant, but it can only point in the general direction of the fault.
It doesn't mean necessarily that the first assumption is correct