I'm guessing you have other stuff running on that old P4 computer.
The general advice is to dedicate the PC to Mach. This is what I do when I set up a PC for Mach3.
Format the hard drive.
Install XP in "Standard PC" mode.
I don't bother with any XP updates, my ethernet is not even enabled.
Install motherboard drivers
Use an independent graphics card
Install graphics card drivers
Go through XP optimisation for Mach3
Then I go a bit "nerdy" and un-install any Windows programs not needed.
Then I go into "services.msc" and completely disable any Windows services that are not needed.
I even go into the bios and disable anything I think is not needed.
Install Mach3
In a nutshell, I go silly to try and ensure the PC is doing nothing but running Mach3. Windows is a multi-tasking system and there are plenty of reports of Mach3 not running right when users have other stuff running on the PC.
I do admit I go a lot further than most other users in this regard.
See also this post:
http://www.machsupport.com/forum/index.php/topic,30566.msg212264.html#msg212264Keith.