Yes, I have read that new lifters can get bad quickly. But not sure the lifters are the issue. It may be that lifter bore ware get worse once new lifters are used as they alter the ware pattern of the old lifter.

Hmm thats a possibility. Why the car ran good for a 5k miles, and than a few days after I changed it is still a bit of a mystery. Unless the new lifter broke in and the tolerances increased.

If my lifter bore is in fact worn, that doesn't seem like an easy fix at all. Either a single solid lifter, or some kind of sleeve. Whether it would be cost effective or if someone sells a solid lifter kit that would work for one lifter, I'm not sure.

Can't see why a single solid lifter will not do. As long as the gap is adjusted correctly it should be fine.

That sounds ideal. Where could I start looking for a single solid lifter for this car?

Done a google search and found this link.
SOLID LIFTER 16V at LINDSEY RACING - Your Porsche Performance Parts Center
You may be able to contact them to just get one lifter.

Update:

Spoke to Greg Brown at Precision motorwerks. Extremely helpful guy probably the most genuine around when it comes to cars like ours, and obviously the 928.

I think I'll be going the route of removing the heading and having a sleeve installed in the lifter bore, as per his suggestion. I'd rather not fiddle with a solid lifter or LR.

Will post some pictures of the bores when I take out the cams again ASAP.

Sound like a good plan. Looking forward to see your progress.