Every now and then at the start of a toolpath the machine will move to the center and then go and cut a shifted distance off in the y position.  It's not a machine problem and I start the machine very slowly because I thougt the initial movement may have made it skip a step.  The machine can move fast during long cuts with no problem it is just when starting a new cut.  I can run the same path several times with no problems then I get it again.  This is very frustrating and makes a lot of scrap.    Last night I ran a cut for 1 1/2hrs then started a profile path that started off and ruined it.  The computer running the machine is dedicated and has no internet or screen saver just Mach 3.  Any ideas would be greatly appreciated.

Do the DROs and the Toolpath show the axis  being in the wrong position?
Hood

I haven't noticed that it looked odd but I will have to pay closer attention. 

That will be a good indication as if the Toolpath and the DRO show the tool is out of position then Mach has sent it there. If the Toolpath and DROs show the position to be where the code is then it is less likely Mach is the culprit, but not impossible
Hood

I have had issues after verifying x and y zero prior to starting the run.  I actually mark an offset zeropoint on the machine so I can reset zero between runs and still have issues.  I have also run a pattern with the z high and not cutting multiple times and had it go off after the 3rd or 4th time.

Presume from that you dont have home switches so I am wondering if you ever do a RefAll ?
As said previously thiough, what the toolpath is showing will be a big help to trying to find the problem, so next time it happens please have a look and see if the toolpath and DROs show where the tool actually is or if they show where it should be.

Hood

I caught it before wrecking another part.  After running the finish path correctly the machine decided to move to a random spot and loose both x and y  positions.  This isn't due to overtravel.  x was off by 6.5" and y was 10".  If I didn't notice this before a tool change it would have been bad.  This seems to be a totally random occurance.

So did the DRO and Toolpath show this out of position?
Also please attach our xml and any code that makes it happen and I will try and simulate.
Hood

I managed a few good days with no problems.  At the end of the toolpath run the machine just moved to the odd position and it did show on the screen.  I don't believe it showed up in the toolpath.  I am pretty new to this and am not sure about saving an xml.  I will try and pay more attention to it and try to root cause it more.  At this point it looks like the problem is at the end of the run and not the beginning.  I will send files the next time it happens.  I appreciate the help.  I have been slow to respond because I have been really busy at work and haven't been able to play with my machine much.