Are you on a wireless network? Wireless pci cards take priority so they have a tendancy to suck the life from your audio device. This is just a guess but a possibility.

I turn off the internal wireless when tracking and playback.

we'll probably need more info if we're to help... which interface??? when it drops out is the FW light still on??? are you playing these filess off the net??

Now I get the dropouts when playing my own record music as well.

Some more info:
The interface is an Alesis Multimix Firewire 8, I don't have a firewire port, so I bought a Firewire PC card, when recording the first tracks of a song in Cubase le (without any other tracks recorded) the unit works fine, but when I want to overdub or playback the tracks, I get many complete dropouts of sound. I am using windows xp, I have the multimix set to the default playback, recording device, etc... I've recorded the device using AMS and have no problems, I would use acid music studio for all my recording, but it only allows one track to be record at once. All these dropouts make it impossible to do any overdubs! Anybody know why this happens? I shut off the internal wireless which did no help.

what fw card??? some interfaces have problems with chipsets other than TI's for instance my tascam is that way... also you may need to do some of the optimization tricks... turn off background services etc... how about the drivers???latest and greatest???? sometimes a complete wipe and start over does the job...