Dell Dimension 3000 latency issues

Not that I know of, I think 12.04 is as much of a dead end as 10.4, as far as Linuxcnc is concerned. In fact I'm not sure that it even counts as being fully supported. Linuxcnc switched to Debian rather than go on with Ubuntu.

The time line I just looked up has support for 12.04 end late in 2017 (ubuntu support, not linuxcnc). They all end sometime but that is reasonably far off (IMHO). So no issues with loading a few packages I may find I need or want.

But I am not tied to Ubuntu. Should I be looking at Debian? Any issues with running the latest Debian version on an old P4? My understanding (which is very limited) is that 10.04 is a suggested OS for such a PC.

It would likely work better than Ubuntu 12.04. The only P4 machine I currently have running Linuxcnc was installed using Ubuntu 10.04 years ago (started with EMC2 v 2.3) and is currently running Linuxcnc 2.7.4 on that original Ubuntu install. Sometime I should plug in the current live iso and see how it runs there.

Edit:
I just took a USB drive with the Linuxcnc Wheezy image on it out to the machine and plugged it in. Short term Latency times were about the same as I had with Ubuntu 10.04.

Ok. I loaded 10.04 and updated to LinuxCNC 2.7. My latency is MUCH better - to the point of being usable at 25,000. But I would like to reduce it more since my driver is microstepping and I have motor reductions. I see numbers under 10,000 at the start that then jump up. What are my next steps? I have read the warnings about messing with SMI. How do I know, or at least have good guess that breaking my PC is unlikely?

The P4 I have was only able to do about a 30.000us base thread, when I was still software stepping. (It has a Mesa 5i25+7i85s now)

Sometimes adding or having the right graphics card can help with these old P4s.
My machine has step/dir servos and not step motors, so having fast high resolution stepping was helpful. But on most step motor systems there usually isn't much point in having micro-stepping set higher than about 4.