5i25 Deterioration?

That sounds good. Any idea on an approximate turnaround time? Debating whether to buy a second/backup 6i25 to use in the meantime (which would transfer to a future 5-axis gantry router build, so I'm less concerned with that cost). If it's likely a week or less then there's probably not much point though.

What should I do to get the ball rolling on the evaluation?

I grabbed the 6i25 just to be on the safe side, should arrive Monday.

I also found a refurb i3 Dell for $38 shipped on eBay, so I nabbed it. That should take care of any amd64 issues (which, while apparently not reproducible, I have seen brought up elsewhere). I also threw in one of the ASMedia 1083-chipset-based PCIe-PCI risers mentioned in the other thread since apparently it works at least well enough for testing, maybe production. I'll have to deal with form factor and mounting shield issues at some point, but for now it should all at least go together.

If the 5i25 is still not liking either Windows or x64 (which is still "amd64", but...y'know) architecture, then I'll email the sales address on the Mesa store.

(Or Tuesday, rather, I don't keep up with all these holidays...)

I haven't ever used Intel for Linuxcnc in 15+ years. What amd64 issues are there with LCNC? Don't mean to hijack the thread or argue. Just wondering.

So far the only AMD system that worked properly with Linuxcnc was a HP DC5750 if I remember correctly, it has already done over four years of every day use without a single issue.
It did have issues with serial ATA DVD recorders, readers were OK! This issue was posted on their web site.
AMD has a nasty habit of not caring much about some rules in electronic and PC world, like some of their high end graphic cards drawing way over 75W of allowed power from the PCI-express slot. I had some that drew 9A, or 108W! That tended to render some cheaper mother boards useless!
Can not offer more info as I did not use AMD since that mentioned above.

We used AMD for all the RTAI work in the past 10 years. The only issues we ever had were BIOS problems or left out features such as "Turn Off Power Management" from some of the motherboard vendors like Biostar, ASRock and MSI.