[Heybuck]

My money says the problem with the eBay drum is there is corrosion between the iron liner and the alloy outer.
You can't see it but it acts as an insulator so the heat from the iron liner cannot transfer as quickly into the alloy. The drums effectively go slightly oval as the heat up with normal use. Fine for the first mile or so then get progressively worse. It will judder and grab as you describe.
If you remove the offending drum you will probably see a darker shadow on the liner. That's the problem zone under which the corrosion lies. Relining it is your only option.

I spent a FORTUNE on my 8 lugs. They were well worn when I bought them, I paid a huge bill to air freight them to Roger Rheil to reline them. And another huge bill to return them.He told me a bearing had spun in one drum and he sold me a replacement. They were OK for a while but I was never overly happy with them so when the front drums were repopped by???? (Can't recall his name right now), I bought a set. First up with the repop ones were there were no lugs installed. I had to get them pressed out of my old drums and into the repops. Then the brake guy I use and respect, told me that one new drum was out of round. He had to machine it out about 100thou to be round. And machine the other good drum to match. So machined out to their maximum wear limit and they were brand new.
About 2 years ago one of my rear drums went out of round for who knows what reason??? I just had had enough of them, put original drums and rims on the car, bought a set of NOS spoke type hub caps and have never been happier. Smooth braking every time.

I'll probably look into putting the 8 lugs back on the car one day after I sort out my bad rear drums but for the present, no 8 lugs works for me