Roy, I doubt the problems you are having are related to an underspec PC, it sounds like a cutter comp problem that some Mach releases are known to have, perhaps some one else can speak on whether they have been fully eliminated from the current lockdown version.
Regarding switching to the new version of Mach3 with your underspec PC, I'm in the same boat, I'm using a 667Mhz system to run an early 3.0 version of Mach and I'm concerned about whether it will have enough beans to run the lastest lockdown version. I asked Brian about this and he claimed that the newest version uses less CPU than at least the version I'm running, which was a release that came shortly after the 2.63 lockdown version you are are running.
I'd like to hang on to my current PC if possible, its been very reliable running Mach. If the current "new" lockdown continues to look good (as it does now, the bug reports in the forum seem pretty low) I'm going to be swapping to it to see what happens. I'm going to keep a backup copy of my current Mach folder in case I need to go back to the earlier version.
I'm probably going to make the swap in a week or 2, I'm waiting for a replacement lube hose to come in for my machine, after I make the swap I'll post the results back here. Please post back how it goes for you if you make the swap in the short time.
Paul T.